SQL Data Definition Language (DDL)

The Data Definition Language (DDL) part of SQL permits database tables to be created or deleted. We can also define indexes (keys), specify links between tables, and impose constraints between database tables.
The most important DDL statements in SQL are:
• CREATE TABLE - creates a new database table
• ALTER TABLE - alters (changes) a database table
• DROP TABLE - deletes a database table
• CREATE INDEX - creates an index (search key)
• DROP INDEX - deletes an index

SQL Data Manipulation Language (DML)

SQL (Structured Query Language) is a syntax for executing queries. But the SQL language also includes a syntax to update, insert, and delete records.
These query and update commands together form the Data Manipulation Language (DML) part of SQL:
• SELECT - extracts data from a database table
• UPDATE - updates data in a database table
• DELETE - deletes data from a database table
• INSERT INTO - inserts new data into a database table

What is SQL?

• SQL stands for Structured Query Language
• SQL allows you to access a database
• SQL is an ANSI standard computer language
• SQL can execute queries against a database
• SQL can retrieve data from a database
• SQL can insert new records in a database
• SQL can delete records from a database
• SQL can update records in a database
• SQL is easy to learn


SQL is an ANSI (American National Standards Institute) standard computer language for accessing and manipulating database systems. SQL statements are used to retrieve and update data in a database. SQL works with database programs like MS Access, DB2, Informix, MS SQL Server, Oracle, Sybase, etc.
Unfortunately, there are many different versions of the SQL language, but to be in compliance with the ANSI standard, they must support the same major keywords in a similar manner (such as SELECT, UPDATE, DELETE, INSERT, WHERE, and others).
Note: Most of the SQL database programs also have their own proprietary extensions in addition to the SQL standard!

How can I restore passwords ?

grant connect to SCOTT identified by passwords GOBBLEDYGOOK;

How do I order a union ?

select DEPTNO, DNAME from DEPT
union
select EMPNO, ENAME from EMP
order by 2;

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.

How do I get a top ten ?

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;

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.

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.

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.

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.

How can I get a name for a temporary table that will not clash ?

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
);

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.

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
);

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
);

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.

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.

How can I change my Oracle password?

Issue the following SQL command: ALTER USER IDENTIFIED BY
/
From Oracle8 you can just type "password" from SQL*Plus, or if you need to change another user's password, type "password user_name".

Can one rename a column in a table?

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 as select * from t1_base;

2. -- Recreate the table with correct column names...
create table t2 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;

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 from t1_base;

2. SQL> create table t2 as select from t1;
SQL> drop table t1;
SQL> rename t2 to t1;

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.

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;

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

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.

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 <>

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;

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.

Can one retrieve only rows X to Y from a table?

SELECT * FROM ( SELECT ENAME,ROWNUM RN FROM EMP WHERE ROWNUM<101)where>

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).

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);>

Can one retrieve only the Nth row from a table?

SELECT * FROM t1 a WHERE n = (SELECT COUNT(rowid)
FROM t1 b WHERE a.rowid >= b.rowid);


SELECT * FROM (SELECT ENAME,ROWNUM RN FROM EMP WHERE ROWNUM < style=""> WHERE RN = 100;


SELECT * FROM emp WHERE rownum=1 AND rowid NOT IN (SELECT rowid FROM emp WHERE rownum <>

there is no explicit row order in a relational database. However, this query is quite fun and may even help in the odd situation.

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;

How does one count different data values in a column?

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;

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)

How does one get the time difference between two date columns?

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.

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.

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);

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;

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

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

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