DELETE

Introduction#

The DELETE statement is used to delete records from a table.

Syntax#

1. DELETE FROM TableName [WHERE Condition] [LIMIT count]

DELETE certain rows with WHERE

This will delete all rows that match the WHERE criteria.

DELETE FROM Employees
WHERE FName = 'John'

DELETE all rows

Omitting a WHERE clause will delete all rows from a table.

DELETE FROM Employees

See TRUNCATE documentation for details on how TRUNCATE performance can be better because it ignores triggers and indexes and logs to just delete the data.

TRUNCATE clause

Use this to reset the table to the condition at which it was created. This deletes all rows and resets values such as auto-increment. It also doesn’t log each individual row deletion.

TRUNCATE TABLE Employees

DELETE certain rows based upon comparisons with other tables

It is possible to DELETE data from a table if it matches (or mismatches) certain data in other tables.

Let’s assume we want to DELETEdata from Source once its loaded into Target.

DELETE FROM Source
WHERE  EXISTS ( SELECT 1 -- specific value in SELECT doesn't matter
               FROM Target
               Where Source.ID = Target.ID )

Most common RDBMS implementations (e.g. MySQL, Oracle, PostgresSQL, Teradata) allow tables to be joined during DELETE allowing more complex comparison in a compact syntax.

Adding complexity to original scenario, let’s assume Aggregate is built from Target once a day and does not contain the same ID but contains the same date. Let us also assume that we want to delete data from Source only after the aggregate is populated for the day.

On MySQL, Oracle and Teradata this can be done using:

DELETE FROM Source
WHERE  Source.ID = TargetSchema.Target.ID
       AND TargetSchema.Target.Date = AggregateSchema.Aggregate.Date

In PostgreSQL use:

DELETE FROM Source
USING  TargetSchema.Target, AggregateSchema.Aggregate
WHERE  Source.ID = TargetSchema.Target.ID
       AND TargetSchema.Target.DataDate = AggregateSchema.Aggregate.AggDate

This essentially results in INNER JOINs between Source, Target and Aggregate. The deletion is performed on Source when the same IDs exist in Target AND date present in Target for those IDs also exists in Aggregate.

Same query may also be written (on MySQL, Oracle, Teradata) as:

DELETE Source
FROM   Source, TargetSchema.Target, AggregateSchema.Aggregate
WHERE  Source.ID = TargetSchema.Target.ID
       AND TargetSchema.Target.DataDate = AggregateSchema.Aggregate.AggDate

Explicit joins may be mentioned in Delete statements on some RDBMS implementations (e.g. Oracle, MySQL) but not supported on all platforms (e.g. Teradata does not support them)

Comparisons can be designed to check mismatch scenarios instead of matching ones with all syntax styles (observe NOT EXISTS below)

DELETE FROM Source
WHERE NOT EXISTS ( SELECT 1 -- specific value in SELECT doesn't matter
               FROM Target
               Where Source.ID = Target.ID )