[required reading for high-quality products] sub query of Oracle database in Linux system

The goal is ：

Use subquery , It can be used SQL Statement to solve complex problems .

Concept

Subqueries are embedded in other SQL In the sentence select In the sentence , Also called nested query .

classification ：

1. Single line sub query ： A subquery that returns only one record .

The operator	meaning
=	equal to
>	greater than
>=	greater than or equal to
<	less than
<=	less than or equal to
<>	not equal to

Inquire about 2012 year 1 Records of monthly water consumption greater than the average
analysis ：
First step , Find out 2012 year 1 Average monthly water consumption （ Think of this as a whole , Distinguish by parentheses ）;

The second step , combined , Find out 2012 year 1 The monthly water consumption is greater than the value in the first step
1） Inquire about 2012 year 1 Format of monthly water consumption greater than a certain value