In the database structured query language (SQL), the DELETE statement removes one or more records from a table. A subset may be defined for deletion using a condition, otherwise all records are removed. Some DBMSs, like MySQL, allow deletion of rows from multiple tables with one DELETE statement (this is sometimes called multi-table DELETE).
DELETE statement follows the syntax:
Any rows that match the
WHERE condition will be removed from the table. If the
WHERE clause is omitted, all rows in the table are removed. The
DELETE statement should thus be used with caution.
DELETE statement does not return any rows; that is, it will not generate a result set.
DELETE statement can cause triggers to run that can cause deletes in other tables. For example, if two tables are linked by a foreign key and rows in the referenced table are deleted, then it is common that rows in the referencing table would also have to be deleted to maintain referential integrity.
Delete rows from table pies where the column flavour equals Lemon Meringue:
DELETE FROM pies WHERE flavour='Lemon Meringue';
Delete rows in trees, if the value of height is smaller than 80.
DELETE FROM trees WHERE height < 80;
Delete all rows from mytable:
DELETE FROM mytable;
Delete rows from mytable using a subquery in the where condition:
DELETE FROM mytable WHERE id IN ( SELECT id FROM mytable2 );
Delete rows from mytable using a list of values:
DELETE FROM mytable WHERE id IN ( value1, value2, value3, value4, value5 );
Example with related tables
Suppose there is a simple database that lists people and addresses. More than one person can live at a particular address and a person can live at more than one address (this is an example of a many-to-many relationship). The database only has three tables, person, address, and pa, with the following data:
The pa table relates the person and address tables, showing that Joe, Bob and Ann all live at 2001 Main Street, but Joe also takes up residence on Pico Boulevard.
In order to remove joe from the database, two deletes must be executed:
DELETE FROM person WHERE pid=1; DELETE FROM pa WHERE pid=1;
To maintain referential integrity, Joe's records must be removed from both person and pa. The means by which integrity is sustained can happen differently in varying relational database management systems . It could be that beyond just having three tables, the database also has been set up with a trigger so that whenever a row is deleted from person any linked rows would be deleted from pa. Then the first statement:
DELETE FROM person WHERE pid=1;
would automatically trigger the second:
DELETE FROM pa WHERE pid=1;
Deleting all rows from a table can be very time consuming. Some DBMS offer a TRUNCATE TABLE command that works a lot quicker, as it only alters metadata and typically does not spend time enforcing constraints or firing triggers.
DELETE only deletes the rows. For deleting a table entirely the DROP command can be used.
- "SQL Delete Statement". w3schools.com.