MySQL add index MySQL How to create an index

1. Add primary key (primary key index)
Mysql>alter TABLE ' table_name ' ADD PRIMARY KEY (' column ')
2. Add unique (unique index)
Mysql>alter TABLE ' table_name ' ADD UNIQUE (
' Column '
)
3. Add index (normal index)
Mysql>alter TABLE ' table_name ' ADD INDEX index_name (' column ')
4. Add fulltext (full-text index)
Mysql>alter TABLE ' table_name ' ADD fulltext (' column ')
5. Adding Multi-column indexes
Mysql>alter TABLE ' table_name ' ADD INDEX index_name (' column1 ', ' column2 ', ' column3 ')

Here's a more detailed approach

In MySQL, you can use the ALTER TABLE SQL statement to add an index to a field in a table.

The basic syntax for adding an index to a field in a table by using the ALTER TABLE statement is:
ALTER table < table name > ADD INDEX (< field >);

Let's try adding an index to the T_name field in test.

Mysql> ALTER TABLE test add index (T_NAME);
Query OK, 0 rows affected (0.17 sec)
records:0 duplicates:0 warnings:0

After successful execution, let's take a look at the results.

mysql> describe test;
+------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+-------+
| t_id | Int (11) |     YES | |       NULL | |
| T_name | varchar (50) | NO | MUL |       NULL | |
| T_password | char (32) |     YES | |       NULL | |
| T_birth | Date |     YES | |       NULL | |
+------------+-------------+------+-----+---------+-------+
4 rows in Set (0.00 sec)

As you can see, the key column of the T_name field is changed from the original blank to the Mul. What does this mul mean? A simple explanation: If key is Mul, then the value of the column can be repeated, which is a leading column of a non-unique index (the first column) or a component of a uniqueness index but can contain null values.

MySQL add index MySQL How to create an index