Page 57 - SQL

Chapter 14: DELETE

Introduction

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

Syntax

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

Examples

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 )

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