UNIQUE索引如何在多列上工作？

Question

If I add a UNIQUE index on the email,password columns, will the index check if:

1. Is email unique AND is password unique, or
2. The combination of email,password is unique, as in, it would allow u1@mail.com, 123 and u2@mail.com, 123

Apologies if my thoughts seem abrupt here, I'm having difficulty in expressing them today for some reason. Thanks in advance!

Answer 1

Your second answer is the correct one. The combination of the two fields must be unique.

For #1 you would need a separate unique index on each column.

Answer 2

I'm agree with @redFilter the combination is a better option. But remember that UNIQUE is different that primary key. You can have null values in UNIQUE, if you want to avoid that when you create the table use the "NOT NULL" at the end of each data type.