Mysql define variable by AS or direct

What's the difference between these 2 开发者_如何转开发queries?

select pc.* from payment_coupon pc
select pc.* from payment_coupon AS pc

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Just one word: Readability.

Using AS is optional, like described here in the manual.

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using AS for column name is mandatory , for table name u just write alias with a space

AS is optional for tablename

Eg:

SELECT column1 AS c1 FROM table t // work
SELECT column1 c1 FROM table t // not works
SELECT column1 AS c1 FROM table AS t  //work
SELECT column1 c1 FROM table AS t //  not works