SQL：使用两个索引创建表

Question

I previously created a table using the following SQL code:

CREATE TABLE myTable (
    id_A BIGINT NOT NULL,
    id_B INT NOT NULL,
    some_info varchar(255),
    some_info2 varchar(255),
    PRIMARY KEY (id_A) 
)

In the previous table created, both id_A and id_B would be unique values. I understand that id_A is forced to be unique by the PRIMARY KEY (id_A) code, which is perfect (which also indexes it), however id_B isn't

Here is where I am confused. id_B will also be unique, however I am not sure how to force it to be unique in the table,and how to make the database create an index for it so that future queries that use SELECT on this table will have good preformance. I know I can't have two PRIMARY KEYS:

PRIMARY KEY (id_A) 
PRIMARY KEY (id_B) 

How might I go about making an index for id_B as well so that future queries happen efficiently?

Answer 1

You can add a UNIQUE INDEX on the column. While a table can only have one PRIMARY KEY , it can have as many UNIQUE INDEX es as you might need.

ALTER TABLE myTable
ADD UNIQUE INDEX `UI_myTable_idB` (`id_B` );

Answer 2

You can use the MySQL UNIQUE KEY for the column. This will force the values in the id_B column to be unique, but not use it as the primary key column. You can achieve it with this statement:

CREATE TABLE myTable (
id_A BIGINT NOT NULL,
id_B INT NOT NULL,
some_info varchar(255),
some_info2 varchar(255),
PRIMARY KEY (id_A),
UNIQUE KEY (id_b))

This will automatically index the column like you want it to. Primary keys and unique keys in a MySQL table are essentially the same, except a primary key cannot be NULL, and there can be only one primary key column (or combination of primary key columns). There can be any number of unique key columns.