I have an email column that I want to be unique. But I also want it to accept null values. Can my database have 2 null emails that way?
Yes, MySQL allows multiple NULLs in a column with a unique constraint.
CREATE TABLE table1 (x INT NULL UNIQUE); INSERT table1 VALUES (1); INSERT table1 VALUES (1); -- Duplicate entry '1' for key 'x' INSERT table1 VALUES (NULL); INSERT table1 VALUES (NULL); SELECT * FROM table1;
x NULL NULL 1
This is not true for all databases. SQL Server for example only allows a single NULL value in a column that has a unique constraint.