Syntax of DELETE operator
From time to time the task of deleting records from the table arises. For this, SQL provides operators DELETE и TRUNCATE, of which the first option is the most universal and safe.
General structure of a query with an operator DELETE
DELETE FROM table_name [WHERE the_conditions_of_the_limitations];
In the described request structure, optional parameters are specified in square brackets.
If there is no WHERE record selection condition, then all records will be deleted the specified table.
The same operation (deleting all records) can also be done using TRUNCATE statement. It will drop the table and re-create it - this option is much faster, than deleting all records one by one (as is the case with DELETE ) especially for large tables.
General structure of a query with the TRUNCATE statement
TRUNCATE TABLE table_name;
The MySQL query optimizer automatically uses the TRUNCATE statement if the DELETE statement does not contain a WHERE clause or LIMIT constructs.
However, the TRUNCATE statement has several differences.:
- Triggers are not processed, in particular, the delete trigger
- Deletes all rows in a table without writing the deletion of individual rows of data to the transaction log
- Resets the counter of identifiers to the initial value
- To use, you must have edit rights to the table
Deleting records for multi-table queries
If the DELETE request usesJOIN, then it is necessary to specify from which table (s) you want to delete records.
DELETE table_name_1 [, table_name_2] FROM table_name_1 JOIN table_name_2 ON table_name_1.field = table_name_2.field [WHERE the_conditions_of_the_limitations];
For example, we need to delete all reservations for a home that does not have a kitchen. Then the request will look like this:
DELETE Reservations FROM Reservations JOIN Rooms ON Reservations.room_id = Rooms.id WHERE Rooms.has_kitchen = false;
If, in addition to deleting the reservation, we also needed to delete the accommodation, then the query would look like this:
DELETE Reservations, Rooms FROM Reservations JOIN Rooms ON Reservations.room_id = Rooms.id WHERE Rooms.has_kitchen = false;
Syntax of UPDATE operator
Creating and deleting database