Monday, April 2, 2012

Most Useful SQL Queries material



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Use the command to get the output in txt file of a sql query... 
        spool oradata.txt
        select xxxxxxx      from   tab1;
         spool off

Topics
•  What is SQL and where does it come from? 
•  What are the difference between DDL, DML and DCL commands? 
•  How does one escape characters when building SQL queries? 
•  How does one eliminate duplicate rows from a table? 
•  How does one generate primary key values for a table? 
•  How does one get the time difference between two date columns? 
•  How does one add a day/hour/minute/second to a date value? 
•  How does one count different data values in a column? 
•  How does one count/sum RANGES of data values in a column? 
•  Can one retrieve only the Nth row from a table? 
•  Can one retrieve only rows X to Y from a table? 
•  How does one select EVERY Nth row from a table? 
•  How does one select the TOP N rows from a table? 
•  How does one code a tree-structured query? 
•  How does one code a matrix report in SQL? 
•  How does one implement IF-THEN-ELSE in a select statement? 
•  How can one dump/ examine the exact content of a database column? 
•  Can one drop a column from a table? 
•  Can one rename a column in a table? 
•  How can I change my Oracle password? 
•  How does one find the next value of a sequence? 
•  Workaround for snapshots on tables with LONG columns 
•  Where can one get more info about SQL? 

  What are the difference between DDL, DML and DCL commands?
DDL is Data Definition Language statements. Some examples: 
CREATE - to create objects in the database 
ALTER - alters the structure of the database 
DROP - delete objects from the database 
TRUNCATE - remove all records from a table, including all spaces allocated for the records are removed 
COMMENT - add comments to the data dictionary 
GRANT - gives user's access privileges to database 
REVOKE - withdraw access privileges given with the GRANT command 
DML is Data Manipulation Language statements. Some examples: 
SELECT - retrieve data from the a database 
INSERT - insert data into a table 
UPDATE - updates existing data within a table 
DELETE - deletes all records from a table, the space for the records remain 
CALL - call a PL/SQL or Java subprogram 
EXPLAIN PLAN - explain access path to data 
LOCK TABLE - control concurrency 
DCL is Data Control Language statements. Some examples: 
COMMIT - save work done 
SAVEPOINT - identify a point in a transaction to which you can later roll back 
ROLLBACK - restore database to original since the last COMMIT 
SET TRANSACTION - Change transaction options like what rollback segment to use 
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________________________________________
How does one escape special characters when building SQL queries?
The LIKE keyword allows for string searches. The '_' wild card character is used to match exactly one character, '%' is used to match zero or more occurrences of any characters. These characters can be escaped in SQL. Example: 
        SELECT name FROM emp WHERE id LIKE '%\_%' ESCAPE '\';
Use two quotes for every one displayed. Example:
        SELECT 'Franks''s Oracle site' FROM DUAL;
        SELECT 'A ''quoted'' word.' FROM DUAL;
        SELECT 'A ''''double quoted'''' word.' FROM DUAL;
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________________________________________
How does one eliminate duplicates rows from a table?
Choose one of the following queries to identify or remove duplicate rows from a table leaving only unique records in the table: 
Method 1: 
   SQL> DELETE FROM table_name A WHERE ROWID > (
     2    SELECT min(rowid) FROM table_name B
     3    WHERE A.key_values = B.key_values);
Method 2: 
   SQL> create table table_name2 as select distinct * from table_name1;
   SQL> drop table_name1;
   SQL> rename table_name2 to table_name1;
   SQL> -- Remember to recreate all indexes, constraints, triggers, etc on table... 
Method 3: (thanks to Dennis Gurnick) 
   SQL> delete from my_table t1
   SQL> where  exists (select 'x' from my_table t2
   SQL>                 where t2.key_value1 = t1.key_value1
   SQL>                   and t2.key_value2 = t1.key_value2
   SQL>                   and t2.rowid      > t1.rowid);
Note: One can eliminate N^2 unnecessary operations by creating an index on the joined fields in the inner loop (no need to loop through the entire table on each pass by a record). This will speed-up the deletion process. 
Note 2: If you are comparing NOT-NULL columns, use the NVL function. Remember that NULL is not equal to NULL. This should not be a problem as all key columns should be NOT NULL by definition. 
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________________________________________
How does one generate primary key values for a table?
Create your table with a NOT NULL column (say SEQNO). This column can now be populated with unique values: 
        SQL> UPDATE table_name SET seqno = ROWNUM;
or use a sequences generator: 
        SQL> CREATE SEQUENCE sequence_name START WITH 1 INCREMENT BY 1;
        SQL> UPDATE table_name SET seqno = sequence_name.NEXTVAL;
Finally, create a unique index on this column. 
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________________________________________
How does one get the time difference between two date columns?
Look at this example query: 
        select floor(((date1-date2)*24*60*60)/3600)
               || ' HOURS ' ||
               floor((((date1-date2)*24*60*60) -
               floor(((date1-date2)*24*60*60)/3600)*3600)/60)
               || ' MINUTES ' ||
               round((((date1-date2)*24*60*60) -
               floor(((date1-date2)*24*60*60)/3600)*3600 -
               (floor((((date1-date2)*24*60*60) -
               floor(((date1-date2)*24*60*60)/3600)*3600)/60)*60)))
               || ' SECS ' time_difference
        from   ...
If you don't want to go through the floor and ceiling math, try this method (contributed by Erik Wile): 
        select to_char(to_date('00:00:00','HH24:MI:SS') +
                    (date1 - date2), 'HH24:MI:SS') time_difference
        from ...
Note that this query only uses the time portion of the date and ignores the date itself. It will thus never return a value bigger than 23:59:59. 
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________________________________________
How does one add a day/hour/minute/second to a date value?
The SYSDATE pseudo-column shows the current system date and time. Adding 1 to SYSDATE will advance the date by 1 day. Use fractions to add hours, minutes or seconds to the date. Look at these examples: 
        SQL> select sysdate, sysdate+1/24, sysdate +1/1440, sysdate + 1/86400 from dual;

        SYSDATE              SYSDATE+1/24         SYSDATE+1/1440       SYSDATE+1/86400
        -------------------- -------------------- -------------------- --------------------
        03-Jul-2002 08:32:12 03-Jul-2002 09:32:12 03-Jul-2002 08:33:12 03-Jul-2002 08:32:13
The following format is frequently used with Oracle Replication: 
       select sysdate NOW, sysdate+30/(24*60*60) NOW_PLUS_30_SECS from dual;

        NOW                  NOW_PLUS_30_SECS
        -------------------- --------------------
        03-JUL-2002 16:47:23 03-JUL-2002 16:47:53
Here are a couple of examples: 
Description Date Expression
Now SYSDATE
Tomorow/ next day SYSDATE + 1
Seven days from now SYSDATE + 7
One hour from now SYSDATE + 1/24
Three hours from now SYSDATE + 3/24
An half hour from now SYSDATE + 1/48
10 minutes from now SYSDATE + 10/1440
30 seconds from now SYSDATE + 30/86400
Tomorrow at 12 midnight TRUNC(SYSDATE + 1)
Tomorrow at 8 AM TRUNC(SYSDATE + 1) + 8/24
Next Monday at 12:00 noon NEXT_DAY(TRUNC(SYSDATE), 'MONDAY') + 12/24
First day of next month at 12 midnight TRUNC(LAST_DAY(SYSDATE ) + 1)
First day of the current month TRUNC(LAST_DAY(ADD_MONTHS(SYSDATE,-1))) + 1
The next Monday, Wednesday or Friday at 9 a.m TRUNC(LEAST(NEXT_DAY(sysdate,''MONDAY'' ),NEXT_DAY(sysdate,''WEDNESDAY''), NEXT_DAY(sysdate,''FRIDAY'' ))) + (9/24)
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________________________________________
How does one count different data values in a column?
Use this simple query to count the number of data values in a column: 
select my_table_column, count(*)
from   my_table
group  by my_table_column;
A more sophisticated example... 
        select dept, sum(  decode(sex,'M',1,0)) MALE,
                     sum(  decode(sex,'F',1,0)) FEMALE,
                     count(decode(sex,'M',1,'F',1)) TOTAL
        from   my_emp_table
        group  by dept;
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________________________________________
How does one count/sum RANGES of data values in a column?
A value x will be between values y and z if GREATEST(x, y) = LEAST(x, z). Look at this example: 
        select f2,
               sum(decode(greatest(f1,59), least(f1,100), 1, 0)) "Range 60-100",
               sum(decode(greatest(f1,30), least(f1, 59), 1, 0)) "Range 30-59",
               sum(decode(greatest(f1, 0), least(f1, 29), 1, 0)) "Range 00-29"
        from   my_table
        group  by f2;
For equal size ranges it might be easier to calculate it with DECODE(TRUNC(value/range), 0, rate_0, 1, rate_1, ...). Eg. 
        select ename "Name", sal "Salary",
               decode( trunc(f2/1000, 0), 0, 0.0,
                                          1, 0.1,
                                          2, 0.2,
                                          3, 0.31) "Tax rate"
        from   my_table;
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________________________________________
Can one retrieve only the Nth row from a table?
Rupak Mohan provided this solution to select the Nth row from a table: 
SELECT * FROM t1 a
WHERE  n = (SELECT COUNT(rowid)
FROM t1 b
WHERE a.rowid >= b.rowid);
Shaik Khaleel provided this solution: 
SELECT * FROM (
   SELECT ENAME,ROWNUM RN FROM EMP WHERE ROWNUM < 101 )
WHERE  RN = 100;
Note: In this first query we select one more than the required row number, then we select the required one. Its far better than using a MINUS operation. 
Ravi Pachalla provided these solutions: 
SELECT f1 FROM t1
WHERE  rowid = (
SELECT rowid FROM t1
WHERE  rownum <= 10
MINUS
SELECT rowid FROM t1
WHERE  rownum < 10);
SELECT rownum,empno FROM scott.emp a
GROUP BY rownum,empno HAVING rownum = 4;
Alternatively...
        SELECT * FROM emp WHERE rownum=1 AND rowid NOT IN
           (SELECT rowid FROM emp WHERE rownum < 10);
Please note, there is no explicit row order in a relational database. However, this query is quite fun and may even help in the odd situation. 
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________________________________________
Can one retrieve only rows X to Y from a table?
Shaik Khaleel provided this solution to the problem: 
SELECT * FROM (
   SELECT ENAME,ROWNUM RN FROM EMP WHERE ROWNUM < 101
) WHERE  RN between 91 and 100 ;
Note: the 101 is just one greater than the maximum row of the required rows (means x= 90, y=100, so the inner values is y+1). 
Ravi Pachalla provided this solution: 
SELECT rownum, f1 FROM t1
GROUP BY rownum, f1 HAVING rownum BETWEEN 2 AND 4;

Another solution is to use the MINUS operation. For example, to display rows 5 to 7, construct a query like this: 
        SELECT *
        FROM   tableX
        WHERE  rowid in (
           SELECT rowid FROM tableX
           WHERE rownum <= 7
          MINUS
           SELECT rowid FROM tableX
           WHERE rownum < 5);
Please note, there is no explicit row order in a relational database. However, this query is quite fun and may even help in the odd situation. 
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________________________________________
How does one select EVERY Nth row from a table?
One can easily select all even, odd, or Nth rows from a table using SQL queries like this: 
Method 1: Using a subquery 
        SELECT *
        FROM   emp
        WHERE  (ROWID,0) IN (SELECT ROWID, MOD(ROWNUM,4)
                             FROM   emp);
Method 2: Use dynamic views (available from Oracle7.2): 
        SELECT *
        FROM   ( SELECT rownum rn, empno, ename
                 FROM emp
               ) temp
        WHERE  MOD(temp.ROWNUM,4) = 0;
Method 3: Using GROUP BY and HAVING - provided by Ravi Pachalla 
SELECT rownum, f1
FROM t1
GROUP BY rownum, f1 HAVING MOD(rownum,n) = 0 OR rownum = 2-n
Please note, there is no explicit row order in a relational database. However, these queries are quite fun and may even help in the odd situation. 
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________________________________________
How does one select the TOP N rows from a table?
Form Oracle8i one can have an inner-query with an ORDER BY clause. Look at this example: 
        SELECT *
        FROM   (SELECT * FROM my_table ORDER BY col_name_1 DESC)
        WHERE  ROWNUM < 10;
Use this workaround with prior releases: 
        SELECT *
          FROM my_table a
         WHERE 10 >= (SELECT COUNT(DISTINCT maxcol)
                        FROM my_table b
                       WHERE b.maxcol >= a.maxcol)
         ORDER BY maxcol DESC;
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________________________________________
How does one code a tree-structured query?
Tree-structured queries are definitely non-relational (enough to kill Codd and make him roll in his grave). Also, this feature is not often found in other database offerings. 
The SCOTT/TIGER database schema contains a table EMP with a self-referencing relation (EMPNO and MGR columns). This table is perfect for tesing and demonstrating tree-structured queries as the MGR column contains the employee number of the "current" employee's boss. 
The LEVEL pseudo-column is an indication of how deep in the tree one is. Oracle can handle queries with a depth of up to 255 levels. Look at this example: 
        select  LEVEL, EMPNO, ENAME, MGR
          from  EMP
        connect by prior EMPNO = MGR
          start with MGR is NULL;
One can produce an indented report by using the level number to substring or lpad() a series of spaces, and concatenate that to the string. Look at this example: 
        select lpad(' ', LEVEL * 2) || ENAME ........
One uses the "start with" clause to specify the start of the tree. More than one record can match the starting condition. One disadvantage of having a "connect by prior" clause is that you cannot perform a join to other tables. The "connect by prior" clause is rarely implemented in the other database offerings. Trying to do this programmatically is difficult as one has to do the top level query first, then, for each of the records open a cursor to look for child nodes. 
One way of working around this is to use PL/SQL, open the driving cursor with the "connect by prior" statement, and the select matching records from other tables on a row-by-row basis, inserting the results into a temporary table for later retrieval. 
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________________________________________
How does one code a matrix report in SQL?
Look at this example query with sample output: 
        SELECT  *
        FROM  (SELECT job,
                      sum(decode(deptno,10,sal)) DEPT10,
                      sum(decode(deptno,20,sal)) DEPT20,
                      sum(decode(deptno,30,sal)) DEPT30,
                      sum(decode(deptno,40,sal)) DEPT40
                 FROM scott.emp
                GROUP BY job)
        ORDER BY 1;

        JOB           DEPT10     DEPT20     DEPT30     DEPT40
        --------- ---------- ---------- ---------- ----------
        ANALYST                    6000
        CLERK           1300       1900        950
        MANAGER         2450       2975       2850
        PRESIDENT       5000
        SALESMAN                              5600
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________________________________________
How does one implement IF-THEN-ELSE in a select statement?
The Oracle decode function acts like a procedural statement inside an SQL statement to return different values or columns based on the values of other columns in the select statement. 
Some examples: 
        select decode(sex, 'M', 'Male', 'F', 'Female', 'Unknown')
        from   employees;

        select a, b, decode( abs(a-b), a-b, 'a > b',
                                       0,   'a = b',
                                            'a < b') from  tableX;

        select decode( GREATEST(A,B), A, 'A is greater OR EQUAL than B', 'B is greater than A')...
        select decode( GREATEST(A,B), 
                  A, decode(A, B, 'A NOT GREATER THAN B', 'A GREATER THAN B'), 
                  'A NOT GREATER THAN B')...
Note: The decode function is not ANSI SQL and is rarely implemented in other RDBMS offerings. It is one of the good things about Oracle, but use it sparingly if portability is required. 
From Oracle 8i one can also use CASE statements in SQL. Look at this example: 
        SELECT ename, CASE WHEN sal>1000 THEN 'Over paid' ELSE 'Under paid' END
        FROM   emp;
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________________________________________
How can one dump/ examine the exact content of a database column?
        SELECT DUMP(col1)
        FROM tab1
        WHERE cond1 = val1;

        DUMP(COL1)
        -------------------------------------
        Typ=96 Len=4: 65,66,67,32
For this example the type is 96, indicating CHAR, and the last byte in the column is 32, which is the ASCII code for a space. This tells us that this column is blank-padded. 
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________________________________________
Can one drop a column from a table?
From Oracle8i one can DROP a column from a table. Look at this sample script, demonstrating the ALTER TABLE table_name DROP COLUMN column_name; command. 
Other workarounds: 
1. SQL> update t1 set column_to_drop = NULL;
   SQL> rename t1 to t1_base;
   SQL> create view t1 as select <specific columns> from t1_base;

2. SQL> create table t2 as select <specific columns> from t1;
   SQL> drop table t1;
   SQL> rename t2 to t1;
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________________________________________
Can one rename a column in a table?
From Oracle9i one can RENAME a column from a table. Look at this example: 
ALTER TABLE tablename RENAME COLUMN oldcolumn TO newcolumn;
Other workarounds: 
1. -- Use a view with correct column names...
   rename t1 to t1_base;
   create view t1 <column list with new name> as select * from t1_base;

2. -- Recreate the table with correct column names...
   create table t2 <column list with new name> as select * from t1;
   drop table t1;
   rename t2 to t1;

3. -- Add a column with a new name and drop an old column...
   alter table t1 add ( newcolame datatype );  
   update t1 set newcolname=oldcolname;
   alter table t1 drop column oldcolname;
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________________________________________
How can I change my Oracle password?
Issue the following SQL command: ALTER USER <username> IDENTIFIED BY <new_password>
From Oracle8 you can just type "password" from SQL*Plus, or if you need to change another user's password, type "password user_name". 
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________________________________________
How does one find the next value of a sequence?
Perform an "ALTER SEQUENCE ... NOCACHE" to unload the unused cached sequence numbers from the Oracle library cache. This way, no cached numbers will be lost. If you then select from the USER_SEQUENCES dictionary view, you will see the correct high water mark value that would be returned for the next NEXTVALL call. Afterwards, perform an "ALTER SEQUENCE ... CACHE" to restore caching. 
You can use the above technique to prevent sequence number loss before a SHUTDOWN ABORT, or any other operation that would cause gaps in sequence values. 
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________________________________________
Workaround for snapshots on tables with LONG columns
You can use the SQL*Plus COPY command instead of snapshots if you need to copy LONG and LONG RAW variables from one location to another. Eg: 
COPY TO SCOTT/TIGER@REMOTE     -
CREATE IMAGE_TABLE USING       -
       SELECT IMAGE_NO, IMAGE  -
       FROM   IMAGES;
Note: If you run Oracle8, convert your LONGs to LOBs, as it can be replicated.


Contents of this Page
•  Can I Update From Another Table ? 
•  Can I Remove Duplicate Rows ? 
•  What is ROWID good for ? 
•  What is ROWNUM good for ? 
•  Views 
Questions on views have moved to another page. 
•  Can I implement Tree Structured Queries ? 
•  How can I get information on the row based on group information ? 
•  How can I get a name for a temporary table that will not clash ? 
•  How can I discover what tables, columns, etc are there ? 
•  How can I rename a column ? 
•  Is there a formatter for SQL or PL/SQL ? 
•  How come records for the date I want are missing ? 
•  How can I interpret a two digit year after 2000 ? 
•  What is a cluster ? 
•  How do I control which rollback segment I use ? 
•  How do order a union ? 
•  Who are SCOTT, SYSTEM and SYS ? 
•  How do I avoid blowing rollback segments ? 
•  How can I restore passwords ? 
•  Not Formatted Yet 
________________________________________
Can I Update From Another Table ? 
Yes. For example, if we had a table DEPT_SUMMARY, we could update the number of employees field as follows:
update DEPT_SUMMARY s
set NUM_EMPS = (
select count(1)
from EMP E
where E.DEPTNO = S.DEPTNO
);
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________________________________________
Can I remove duplicate rows ? 
Yes, using the ROWID field. The ROWID is guaranteed unique. There are many variations on this theme, but the logic is to delete all but one record for each key value. 
delete from EMP E
  where not E.ROWID = (
  select min(F.ROWID)
  from EMP F
where  F.EMP_ID = E.EMP_ID
);
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________________________________________
Can I implement Tree Structured Queries ? 
Yes! This is commonly asked by those migrating from non-RDBMS apps. This is definitely non-relational (enough to kill Codd and then make him roll in his grave) and is a feature I have not seen in the competition. 
The definitive example is in the example SCOTT/TIGER database, when looking at the EMP table (EMPNO and MGR columns). The MGR column contains the employee number of the "current" employee's boss. 
You have available an extra pseudo-column, LEVEL, that says how deep in the tree you are. Oracle can handle queries with a depth up to 255. 
select LEVEL, EMPNO, ENAME, MGR 
from EMP 
connect by prior EMPNO = MGR 
start with MGR is NULL; 
You can get an "indented" report by using the level number to substring or lpad a series of spaces and concatenate that to the string. 
select lpad(' ', LEVEL * 2) || ENAME ........ 
You use the start with clause to specify the start of the tree(s). More than one record can match the starting condition. 
One disadvantage of a "connect by prior" is that you cannot perform a join to other tables. Still, I have not managed to see anything else like the "connect by prior" in the other vendor offerings and I like trees. Even trying to doing this programmatically in embedded SQL is difficult as you have to do the top level query, for each of them open a cursor to look for lower level rows, for each of these....... 
soon you blow the cursor limit for your installation. 
The way around this is to use PL/SQL, open the driving cursor with the "connect by prior" statement, and the select matching records from other tables on a row-by-row basis, inserting the results into a temporary table for later retrieval. 
Note that you can't trick Oracle by using CONNECT BY PRIOR on a view that does the join. 
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________________________________________
How can I get information on the row based on group information ? 
Imagine we have the EMP table and want details on the employee who has the highest salary. You need to use a subquery. 
select e.ENAME, e.EMPNO, e.SAL
from EMP e
where e.SAL in (
select max (e2.SAL)
from EMP e2
); 
You could get similar info on employees with the highest salary in their departments as follows 
select e.ENAME, e.DEPTNO, e.SAL
from EMP e
where e.SAL = (
select max (e2.SAL)
from EMP e2
where e2.DEPTNO = e.DEPTNO
); 
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________________________________________
How can I get a name for a temporary table that will not clash ? 
Use a sequence, and use the number to help you build the temporary table name. Note that SQL-92 is developing specific constructs for using temporary tables. 
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________________________________________
How can I discover what tables, columns, etc are there ? 
Oracle maintains a live set of views that you can query to tell you what you have available. In V6, the first two to look at are DICT and DICT_COLUMNS which act as a directory of the other dictionary views. It is a good idea to be familiar with these. 
Not all of these views are accessible by all users. If you are a DBA you should also create private DBA synonyms by running $ORACLE_HOME/rdbms/admin/dba_syn.sql in your account. 
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How can I rename a column ? 
There is no way a column can be renamed using normal SQL. It can be done carefully by the DBA playing around with internal SYS dictionary tables and bouncing the database, but this is not supported. (I have successfully done it in V4 thru V7). Do backup the database first unless you feel brave. I've written a quick and dirty script rncol.sql to do this. If you can't figure out how to use it from the source you definitely should not run it. 
You can use a similar dirty trick for changing ownership of tables if storage space is limited. 
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Is there a formatter for SQL or PL/SQL ? 
There are a number of "beautifiers" for various program languages. The cb and indent programs for the C language spring to mind (although they have slightly different conventions). As far as I know there is no PD formatter for SQL available. 
Given that there are PD general SQL parsers and that the SQL standards are drafted in something close to BNF, maybe someone could base a reformatter based on the grammar. 
Note that you CANNOT use cb and indent with Pro*C as both these programs will screw up the embedded SQL code. 
I have recently heard that Kumaran Systems (see Vendor list) have a Forms PL/SQL and SQL formatter, but I do not now if they have unbundled it. 
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How come records for the date I want are missing ? 
You are trying to retrieve data based on something like: 
SELECT fld1, fld2 FROM tbl WHERE date_field = '18-jun-60' 
You *know* there are records for that day - but none of them are coming back to you. 
What has happened is that your records are not set to midnight (which is the default value if time of day not specified). 
You can either use to_char and to_date functions, which can be a bad move regarding SQL performance, or you can say 
WHERE date_field >= '18-jun-60' AND date_field < '19-jun-60' 
An alternative could be something like 
WHERE date_field between '18-jun-1960'
AND to_date('23:59:59 18-jun-60', 'HH24:......YY')
;
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How can I interpret a two digit year after 2000 ? 
When converting to dates from characters when you only have two characters for the year, the picture format "RR" will be interpreted as the year based on a guess that that date is between 1950 and 2049. 
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What are these V$ tables? 
There are a number of tables/views beginning with V$ that hold gory details for performance monitoring. These are not guaranteed to be stable from minor release to minor release and are for DBAs only. 
There are usually no real underlying tables (unlike SYS.OBJ$) and are dummied up by the RDBMS kernel software in much the same way that UNIX System V.4 dummies up the files in the /proc or /dev/proc directories. 
If you have any code depending on these (and the widely used tools supplied by Oracle but unsupported are in this category) then you need to verify that everything works each time you upgrade your database. And when a major revision changes, all bets are off. 
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How do I get a top ten ? 
This question often gets the response WHERE ROWNUM <= 10 but this will not work (except accidentally) because the ROWNUM pseudocolumn is generated before the ORDER or WHERE clauses come into effect. 
One elegant SQL-only approach (although it will be a bitch on a large table) was suggested by Stowe@aol.com
        select a.ordered_column, a.other_stuff
        from   table_name a
        where  10 > (
                select count(1)
                from   table_name b
                where  b.ordered_column 
                       < a.ordered_column )
        order by a.ordered_columnl; 
I do not believe that straight SQL is the way to go for such problems when you have PL/SQL available. 
My approach is to use PL/SQL instead (in SQL*Plus):
variable tenthsal number 
declare 
n number; 
cursor c1 is
select SAL from EMP order BY SAL desc; 
begin 
open c1; 
for n in 1..10 loop 
fetch c1 into :tenthsal; 
end loop; 
close c1; 
end: 
select * from EMP 
where SAL <= :tenthsal 
order by SAL desc; 
Late news: index descending hint to SQL works if you use a dummy restriction to force use of the index. Needs V7, etc. 
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How do control which rollback segment I use ? 
In SQL, you may need to control the rollback segment used as the default rollback segment may be too small for the required transaction, or you may want to ensure that your transaction runs in a special rollback segment, unaffected by others. The statement is as follows: 
SET TRANSACTION USE ROLLBACK SEGMENT segment_name; 
On a related note, if all you are doing are SELECTS, it is worth telling the database of this using the following: 
SET TRANSACTION READ ONLY; 
Both these statements must be the first statement of the transaction. 
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How do I order a union ? 
(Governments around the world have been trying to figure this one out). 
Use the column number. 
Say we are getting a list of names and codes and want it ordered by the name, using both EMP and DEPT tables:
select DEPTNO, DNAME from DEPT
union
select EMPNO, ENAME from EMP
order by 2;
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Who are SCOTT, SYSTEM and SYS ? 
These three users are common in many databases. See the glossary entries under SCOTT, SCOTT and SYS. Another common user/password is PLSQL/SUPERSECRET used for PL/SQL demo stuff. 
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How can I avoid blowing rollback segments ? 
The simple answer is make sure you have them big enough and keep your transactions small, but that is being a smartarse. 
More recent versions of Oracle have an option for the session that you can set that commits every so many DML statements. This is OK except for where you are doing your work in a single statement rather than using PL/SQL and a loop construct. 
Imagine you have a HUGE table and need to update it, possibly updating the key. You cannot update it in one go because your rollback segments are too small. You cannot open a cursor and commit every n records, because usually the cursor will close. You cannot have a number of updates of a few records each because the keys may change - causing you to visit records more than once. 
The solution I have used was to have one process select ROWID from the appropriate rows and pump these (via standard I/O) to another process that looped around reading ROWIDs from standard input, updating the appropriate record and committing every 10 records or so. This was very easy to program and also was quite fast in execution. The number of locks and size of rollback segments required was minimal. 
If you are writing in Pro*C and use MODE=ORACLE, there are ways around it too, but not if you are using MODE=ANSI. 
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How can I restore passwords ? 
OK, so this is really a DBA question, but it is worth putting in here because it involves SQL regardless of interface. 
First, look at the PASSWORD column in DBA_USERS. It looks like gobbledygook because it is an encrypted password. However you can use this if you have saved it somewhere else. Say you want to impersonate a user in a batch run overnight. First stash the gobbledygook password away somewhere, grant connect to the user identified by some password you know and then run your batches using the new known password. 
To restore the password to what it was use the following syntax (which I think is undocumented).
grant connect to SCOTT identified by passwords GOBBLEDYGOOK;
Note especially the S on the end of PASSWORDS. 
Question: What versions are loaded on my machine ? 
SQL> select substr(product,1,15) product,substr(version,1,15) version,substr(status,1,15) status
from product_component_version

PRODUCT         VERSION         STATUS
--------------- --------------- ---------------
NLSRTL          3.4.1.0.0       Production
Oracle8i Enterp 8.1.7.3.0       Production
PL/SQL          8.1.7.3.0       Production
TNS for 32-bit  8.1.7.3.0       Production

also,

SQL> select * from v$version where banner like 'Oracle%';

BANNER
----------------------------------------------------------------
Oracle8i Enterprise Edition Release 8.1.7.3.0 - Production
Question: What Applications/Versions/Patch Levels are loaded on my machine ? 
this query gives more details than the one below 
 
select   substr(a.APPLICATION_NAME,1,60) "Application Name"
,  substr(i.PRODUCT_VERSION,1,4)  "Version"
,  i.PATCH_LEVEL    "Patch Level"
,  i.APPLICATION_ID   "Application ID"
,  i.LAST_UPDATE_DATE   "Last Update"
from   APPS.FND_PRODUCT_INSTALLATIONS  i
,  APPS.FND_APPLICATION_ALL_VIEW  a
where   i.APPLICATION_ID   = a.APPLICATION_ID
--  not all applications update the next field correctly
--  and i.PATCH_LEVEL   like '11i%'
--  these are the applications that concern me most
--  and i.APPLICATION_ID in   ('0','140','260','101','200','275','201','222','185')
order by a.APPLICATION_NAME
Question: What patches are loaded on my machine ? 
 
select patch_name,
       patch_type,
       applied_patch_id,
       rapid_installed_flag,
       maint_pack_level
from   ad_applied_patches
where  patch_name like '%'
order by 1
How do I obtain CCID / Chart of Account data for General Ledger 
 
  select
   substr(gl.code_combination_id,1,5) ccid,
   substr(gl.segment1,1,5) Auth,
   substr(gl.segment2,1,8) Account,
   substr(gl.segment3,1,5) RC,
   substr(gl.segment4,1,5) Func,
   substr(gl.segment5,1,5) Job
   FROM   gl.gl_code_combinations gl
order by code_combination_id
Question: How many transactions exist by GL DATE (period) ? 
select gl_date,count(gl_date) 
from ra_cust_trx_line_gl_dist_All
where account_class = 'REV'
group by gl_date
Question: What is my current GL SET OF BOOKS 
select SET_OF_BOOKS_ID,               
NAME,SHORT_NAME,                     
CHART_OF_ACCOUNTS_ID,           
CURRENCY_CODE,                  
PERIOD_SET_NAME,                
ACCOUNTED_PERIOD_TYPE,
LATEST_OPENED_PERIOD_NAME,                                
substr(DESCRIPTION,1,30) description from gl_sets_of_books
Question: What DB version is on my machine ? 
 
SQL> VARIABLE VERSION VARCHAR2(50)
SQL> VARIABLE COMPATIBILITY VARCHAR2(50)
SQL> EXEC DBMS_UTILITY.DB_VERSION(:VERSION,:COMPATIBILITY)
 
PL/SQL procedure successfully completed.
 
SQL> PRINT VERSION
 
VERSION
----------------------------------------
8.0.4.0.0
 
SQL> PRINT COMPATIBILITY
 
COMPATIBILITY
----------------------------------------
8.0.0
Question: What machine or instance am I using ?? 
SQL>   Select name from v$database;

NAME
---------
ARGP
               also you can use this:
SQL> select sys_context('USERENV','DB_NAME') AS instance from dual;

INSTANCE
---------------------------------------------------------------------
ARGP

This variation will give you the machine name you are running on: 

SQL> Select sys_context('USERENV','TERMINAL') from dual;

SYS_CONTEXT('USERENV','TERMINAL')
--------------------------------------------------------------
AR0669

Question: How can I retrieve a random number ? 
SQL> select dbms_random.random from dual;

    RANDOM
----------
 495129087
Question: What patches are loaded on our machine ? 
SQL> select * from AD_APPLIED_PATCHES 
Question: Am I setup for Multi-Org ? 
 select multi_org_flag from fnd_product_groups;
Useful DATE output SQL 
This format is yymmdd = year month day | hh24mi = 24 hour clock and minutes 
select to_char(sysdate,'yymmddhh24mi')
 from dual
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