Something-mysql today.

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I have something to play with today, how much can I learn?

1. MySQL

-----subquery and Connection (ii)-----

----Sub-query

A subquery, which is a SELECT clause that appears within other SQL statements

Note: A subquery is nested inside a query and must always appear in parentheses; it can contain multiple keywords or conditions; the outer query can be select,insert,update,set or do

The return of a subquery, which can be a scalar, a row, a column, or a subquery

Can be divided into three categories: the use of comparison operators =,>=, etc.; Not in; EXIST

----Using comparison operators to implement subqueries

--

Mysql> SELECT goods_id,goods_name,goods_price from Tdb_goods WHERE goods_price>=5391.30;

The numbers here can be used without a tube, which is an average of the previous gains.

So if you want to directly get the result of this average participation, write it with a subquery

Mysql> Select Goods_id,goods_name,goods_price from Tdb_goods WHERE goods_price>=, select ROUND (AVG (Goods_price), 2) from Tdb_goods);

As you can see here, the subquery is in parentheses, using the >=

--

When a subquery returns multiple results, and the main query is less than that, you can SOME the ALL keyword with any

Where any is the same as some, it is good to satisfy one of them, specifically which one depends on what operator is (e.g. > is greater than the minimum value).

All requests to meet all return requirements

Usage is written in front of sub-query parentheses

----subquery raised by in or not

In fact, the in and =all operators are equivalent, not in and =! All equivalent

Usage is similar

----EXIST

Returns a Boolean value with very little

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