mysql tutorial: mysql create and delete index

Create and delete indexes
Index creation can be done in the CREATE TABLE statement, or you can use CREATE INDEX or ALTER TABLE alone to add indexes to the table. Dropping an index can be accomplished using the ALTER TABLE or DROP INDEX statement.
(1) Use the ALTER TABLE statement to create an index.
The syntax is as follows:
alter table table_name add index index_name (column_list);
alter table table_name add unique (column_list);
alter table table_name add primary key (column_list);
It includes 3 types: ordinary index, UNIQUE index and PRIMARY KEY index The format for creating an index. table_name is the name of the table to which the index is to be added. column_list indicates which columns to index. If there are multiple columns, separate them with commas. The index name index_name is optional. By default, MySQL will assign a name based on the first index column. Additionally, ALTER TABLE allows multiple tables to be altered in a single statement, so multiple indexes can be created simultaneously.
An example of creating an index is as follows:
mysql> use tpsc
Database changed
mysql> alter table tpsc add index shili (tpmc) ;
Query OK, 2 rows affected (0.08 sec)
Records: 2 Duplicates: 0 Warnings: 0
(2) Use the CREATE INDEX statement to add an index to the table.
Ability to add both normal index and UNIQUE index. The format is as follows:
create index index_name on table_name (column_list);
create unique index index_name on table_name (column_list);
Note: table_name, index_name and column_list have the same meaning as in the ALTER TABLE statement, and the index name is not optional. In addition, you cannot use the CREATE INDEX statement to create a PRIMARY KEY index.
(3) Delete index.
Deleting an index can be achieved using the ALTER TABLE or DROP INDEX statement. DROP INDEX can be processed as a statement inside ALTER TABLE, and its format is as follows:
drop index index_name on table_name;
alter table table_name drop index index_name;
alter table table_name drop primary key;
Among them, in the previous two statements, The index index_name in table_name is deleted. In the last statement, it is only used to delete the PRIMARY KEY index, because a table can only have one PRIMARY KEY index, so there is no need to specify the index name. If no PRIMARY KEY index is created, but the table has one or more UNIQUE indexes, MySQL drops the first UNIQUE index.
If a column is deleted from the table, the index will be affected. For a multi-column index, if one of the columns is deleted, the column will also be deleted from the index. If you delete all the columns that make up the index, the entire index will be deleted.
The operation of deleting the index, such as the following code:
mysql> drop index shili on tpsc;
Query OK, 2 rows affected (0.08 sec)
Records: 2 Duplicates: 0 Warnings: 0
This statement deletes the previously created name Index for "shili".

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