mysql 唯一索引与null.md

create table tb (
  a int,
  b int,
  c int,
  unique index (a,b,c)
);

insert into tb(a,b,c) values (null,null,null); -- ok
insert into tb(a,b,c) values (null,null,null); -- still ok
insert into tb(a,b,c) values (null,null,null); -- still ok
insert into tb(a,b,c) values (1,null,null); -- ok
insert into tb(a,b,c) values (1,2,null); -- ok
insert into tb(a,b,c) values (1,2,3); -- ok
insert into tb(a,b,c) values (1,null,null); -- SHOULD FAIL, BUT DOESN'T
insert into tb(a,b,c) values (1,2,null); -- SHOULD FAIL, BUT DOESN'T
insert into tb(a,b,c) values (1,2,3); -- fails correctly

A UNIQUE index creates a constraint such that all values in the index must be distinct. An error occurs if you try to add a new row with a key value that matches an existing row. For all engines, a UNIQUE index permits multiple NULL values for columns that can contain NULL. If you specify a prefix value for a column in a UNIQUE index, the column values must be unique within the prefix.