Oracle DBA FAQ - Understanding SQL DDL Statements

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By: FYIcenter.com

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How To Delete a Column in an Existing Table?

If you have an existing column in a table and you need that column any more, you can delete it with ALTER TABLE ... DROP COLUMN statement. Here is an example SQL script:

SQL> CREATE TABLE emp_dept_90 
  2  AS SELECT * FROM employees WHERE department_id=90;
Table created.

SQL> SELECT last_name FROM emp_dept_90;
LAST_NAME                
-------------------------
King                     
Kochhar                  
De Haan                  

SQL> ALTER TABLE emp_dept_90 DROP COLUMN last_name;
Table altered.

SQL> SELECT last_name FROM emp_dept_90;
ERROR at line 1:
ORA-00904: "LAST_NAME": invalid identifier

As you can see the column "last_name" is gone.

How To Drop an Existing Table?

If you want to delete an existing table and its data rows, you can use the DROP TABLE statement as shown in this script:

SQL> connect HR/fyicenter
Connected.

SQL> CREATE TABLE emp_dept_10
  2  AS SELECT * FROM employees WHERE department_id=10;
Table created.

SQL> DROP TABLE emp_dept_10;
Table dropped.

Be careful, when you use the DROP TABLE statement. All data rows are gone too.

How To Create a Table Index?

If you have a table with a lots of rows, and you know that one of the columns will be used often a search criteria, you can add an index for that column to in improve the search performance. To add an index, you can use the CREATE INDEX statement as shown in the following script:

CREATE TABLE tip (id NUMBER(5) PRIMARY KEY,
  subject VARCHAR(80) NOT NULL,
  description VARCHAR(256) NOT NULL,
  create_date DATE DEFAULT (sysdate));
Table created.
  
CREATE INDEX tip_subject ON tip(subject);
Index created.

How To Rename an Index?

Let's say you have an existing index, and you don't like its name anymore for some reason, you can rename it with the ALTER INDEX ... RENAME TO statement. Here is an example script on how to rename an index:

CREATE TABLE student (id NUMBER(5) PRIMARY KEY,
  first_name VARCHAR(80) NOT NULL,
  last_name VARCHAR(80) NOT NULL,
  birth_date DATE NOT NULL,
  social_number VARCHAR(80) UNIQUE NOT NULL);
Table created.
  
SELECT index_name, table_name, uniqueness 
  FROM USER_INDEXES WHERE table_name = 'STUDENT';
INDEX_NAME              TABLE_NAME            UNIQUENES
----------------------- --------------------- ---------
SYS_C004153             STUDENT               UNIQUE
SYS_C004154             STUDENT               UNIQUE

ALTER INDEX SYS_C004153 RENAME TO student_pk;
Statement processed.

SELECT index_name, table_name, uniqueness 
  FROM USER_INDEXES WHERE table_name = 'STUDENT';
INDEX_NAME              TABLE_NAME            UNIQUENES
----------------------- --------------------- ---------
STUDENT_PK              STUDENT               UNIQUE
SYS_C004154             STUDENT               UNIQUE

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