SQL Delete Statement

The DELETE Statement is used to delete rows from a table.

The Syntax of a SQL DELETE statement is:

DELETE FROM table_name [WHERE condition];

·         table_name -- the table name which has to be updated.

NOTE:The WHERE clause in the sql delete command is optional and it identifies the rows in the column that gets deleted. If you do not include the WHERE clause all the rows in the table is deleted, so be careful while writing a DELETE query without WHERE clause.

For Example: To delete an employee with id 100 from the employee table, the sql delete query would be like,

DELETE FROM employee WHERE id = 100;

To delete all the rows from the employee table, the query would be like,

DELETE FROM employee;

SQL TRUNCATE Statement

The SQL TRUNCATE command is used to delete all the rows from the table and free the space containing the table.

Syntax to TRUNCATE a table:

TRUNCATE TABLE table_name;

For Example: To delete all the rows from employee table, the query would be like,

TRUNCATE TABLE employee;

Difference between DELETE and TRUNCATE Statements:

DELETE Statement: This command deletes only the rows from the table based on the condition given in the where clause or deletes all the rows from the table if no condition is specified. But it does not free the space containing the table.

TRUNCATE statement: This command is used to delete all the rows from the table and free the space containing the table.

SQL DROP Statement:

The SQL DROP command is used to remove an object from the database. If you drop a table, all the rows in the table is deleted and the table structure is removed from the database. Once a table is dropped we cannot get it back, so be careful while using RENAME command. When a table is dropped all the references to the table will not be valid.

Syntax to drop a sql table structure:

DROP TABLE table_name;

For Example: To drop the table employee, the query would be like

DROP TABLE employee;

Difference between DROP and TRUNCATE Statement:

If a table is dropped, all the relationships with other tables will no longer be valid, the integrity constraints will be dropped, grant or access privileges on the table will also be dropped, if want use the table again it has to be recreated with the integrity constraints, access privileges and the relationships with other tables should be established again. But, if a table is truncated, the table structure remains the same, therefore any of the above problems will not exist.

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The DELETE statement allows you to delete a single record or multiple records from a table.

The syntax for the DELETE statement is:

DELETE FROM table
WHERE predicates;

Example #1 - Simple example

Let's take a look at a simple example:

DELETE FROM suppliers
WHERE supplier_name = 'IBM';

This would delete all records from the suppliers table where the supplier_name is IBM.

You may wish to check for the number of rows that will be deleted. You can determine the number of rows that will be deleted by running the following SQL statement before performing the delete.

SELECT count(*)
FROM suppliers
WHERE supplier_name = 'IBM';

Example #2 - More complex example

You can also perform more complicated deletes.

You may wish to delete records in one table based on values in another table. Since you can't list more than one table in the FROM clause when you are performing a delete, you can use the EXISTS clause.

For example:

DELETE FROM suppliers
WHERE EXISTS
  ( select customers.name
     from customers
     where customers.customer_id = suppliers.supplier_id
     and customers.customer_name = 'IBM' );

This would delete all records in the suppliers table where there is a record in the customers table whose name is IBM, and the customer_id is the same as the supplier_id.

If you wish to determine the number of rows that will be deleted, you can run the following SQL statement before performing the delete.

SELECT count(*) FROM suppliers
WHERE EXISTS
  ( select customers.name
     from customers
     where customers.customer_id = suppliers.supplier_id
     and customers.customer_name = 'IBM' );