SQL Server | How Mysql implements the unique implementation of a table

There are two implementations of unique in the ANSI SQL standard

1. It is possible to insert multiple null values, which means that multiple null values are considered to be distinct.

2, it is only possible to insert a null value, also the main is to say that all the empty values to see the same.

In SQL Server | The second standard is implemented in MySQL.

Example: (Code with SQL Server syntax as an example)

CREATE TABLE tmp (x int primary key, y int);

ALTER TABLE TMP add constraint unique_cons unique (y);

INSERT into TMP (x, Y) VALUES (1,null);

INSERT into TMP (x, Y) VALUES (2,null); This is not going to happen. This means that when the second null is inserted, two null is treated as the same value.

SQL Server | How Mysql implements the unique implementation of a table