Delete (SQL)

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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.[1] Some DBMSs, like MySQL, allow to delete rows from multiple tables with one DELETE statement (this is sometimes called multi-table DELETE).

Usage[edit]

The DELETE statement follows the syntax:

DELETE FROM table_name [WHERE condition];

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.

The DELETE statement does not return any rows; that is, it will not generate a result set.

Executing a 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.

Examples[edit]

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[edit]

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:

person

pid name
1 Joe
2 Bob
3 Ann

address

aid description
100 2001 Main St.
200 35 Pico Blvd.

pa

pid aid
1 100
2 100
3 100
1 200

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[citation needed]. 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;

Related commands[edit]

Deleting all rows from a table can be very time consuming. Some DBMS[clarification needed] 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.

References[edit]

  1. ^ "SQL Delete Statement". w3schools.com.