The DELETE statement is used to delete records from a table.
This will delete all rows that match the
DELETE FROM Employees WHERE FName = 'John'
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.
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
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 )