Delete Records That Are Not Referenced

MySQL Tutorials - Herong's Tutorial Examples

∟Delete Records That Are Not Referenced

This section provides a tutorial example on bulk deletion for records that are not referenced by another table.

There are two options to delete records that are not referenced by another table. For example, deleting all orders that has no order lines can be done by:

1. Using "NOT IN (...)":

DELETE from Orders where ID NOT IT (select OrderID from OrderLines);

2. Using "LEFT JOIN":

DELETE Orders from Orders o LEFT JOIN OrderLines l ON o.ID = l.OrderID 
  where l.CaseID is null;

Let's compare try both options and see their performance difference.

1. Using "NOT IN (...)" to delete records that are not referenced. I see that deleting 6% of 687533 rows takes 1.80 seconds.

mysql> select count(*) from Orders
|   687533 |

mysql> drop Table Orders_Test;
mysql> create Table Orders_Test like Orders;
mysql> insert into Orders_Test select * from Orders; 

mysql> select count(*) from Orders_Test 
  where ID NOT IN (select OrderID from OrderLines);
|    46829 |

mysql> select 46829/687533;
|   0.0681 |

mysql> DELETE from Orders_Test 
  where ID NOT IN (select OrderID from OrderLines);
Query OK, 46829 rows affected (1.80 sec)

mysql> drop Table Orders_Test;

2. Using "LEFT JOIN" to delete records that are not referenced. I see that deleting 6% of 687533 rows takes 2.04 seconds.

mysql> create Table Orders_Test like Orders;
mysql> insert into Orders_Test select * from Orders; 

mysql> select count(*) from Orders_Test o 
    -> LEFT JOIN OrderLines l ON o.ID = l.OrderID where l.OrderID is null
|    46829 |

mysql> DELETE o from Orders_Test o 
    -> LEFT JOIN OrderLines l ON o.ID = l.OrderID where l.OrderID is null
Query OK, 46829 rows affected (2.04 sec)

mysql> select count(*) from Orders_Test 
  where ID NOT IN (select CaseID from CaseMolecules);
|        0 |

mysql> drop Table Orders_Test;