当我们需要根据指定条件从单张表或者多张表中查询数据时，就可以在 SELECT 语句中添加 WHERE 子句，从而过滤掉我们不需要数据。
WHERE子句不仅可以用于SELECT语句中，同时也可以用于UPDATE,DELETE等语句中。
1.语法
SELECT column1, column2, columnNFROM 表名称WHERE [条件]
和AND结合使用
找出 AGE(年龄) 字段大于等于 25，并且 SALARY(薪资) 字段大于等于 65000 的数据：
runoobdb=# SELECT * FROM COMPANY WHERE AGE >= 25 AND SALARY >= 65000;
id | name | age | address | salary
----+-------+-----+----------------------------------------------------+--------
4 | Mark | 25 | Rich-Mond | 65000
5 | David | 27 | Texas | 85000
(2 行记录)
和OR结合使用
找出 AGE(年龄) 字段大于等于 25，或者 SALARY(薪资) 字段大于等于 65000 的数据：
runoobdb=# SELECT * FROM COMPANY WHERE AGE >= 25 OR SALARY >= 65000;
id | name | age | address | salary
----+-------+-----+----------------------------------------------------+--------
1 | Paul | 32 | California | 20000
2 | Allen | 25 | Texas | 15000
4 | Mark | 25 | Rich-Mond | 65000
5 | David | 27 | Texas | 85000
(4 行记录)
和NOT NULL结合使用
在公司表中找出 AGE(年龄) 字段不为空的记录：
runoobdb=# SELECT * FROM company WHERE age IS NOT NULL;
id | name | age | address | salary
----+-------+-----+----------------------------------------------------+--------
1 | Paul | 32 | California | 20000
2 | Allen | 25 | Texas | 15000
3 | Teddy | 23 | Norway | 20000
4 | Mark | 25 | Rich-Mond | 65000
5 | David | 27 | Texas | 85000
6 | Kim | 22 | South-Hall | 45000
7 | James | 24 | Houston | 10000
(7 行记录)
LIKE
在 COMPANY 表中找出 NAME(名字) 字段中以 Pa 开头的数据：
runoobdb=# SELECT * FROM company WHERE name LIKE 'Pa%';
id | name | age | address | salary
----+------+-----+----------------------------------------------------+--------
1 | Paul | 32 | California | 20000
(1 行记录)
在 COMPANY 表中找出 NAME(名字) 字段中以 y结尾的数据：
runoobdb=# SELECT * FROM company WHERE name LIKE '%y';
id | name | age | address | salary
----+-------+-----+----------------------------------------------------+--------
3 | Teddy | 23 | Norway | 20000
(1 行记录)
IN
以下 SELECT 语句列出了 AGE(年龄) 字段为 25 或 27 的数据
runoobdb=# SELECT * FROM company WHERE age IN (25,27);
id | name | age | address | salary
----+-------+-----+----------------------------------------------------+--------
2 | Allen | 25 | Texas | 15000
4 | Mark | 25 | Rich-Mond | 65000
5 | David | 27 | Texas | 85000
(3 行记录)
注意：这个IN不是在25~27之间的意思，是指25和27的意思
比如下面列出年龄为，25,27,32的数据：
runoobdb=# SELECT * FROM company WHERE age IN (25,27,32);
id | name | age | address | salary
----+-------+-----+----------------------------------------------------+--------
1 | Paul | 32 | California | 20000
2 | Allen | 25 | Texas | 15000
4 | Mark | 25 | Rich-Mond | 65000
5 | David | 27 | Texas | 85000
(4 行记录)
NOT IN
找出 AGE(年龄) 字段不为 25 或 27 的数据：
runoobdb=# SELECT * FROM company WHERE age NOT IN(25,27);
id | name | age | address | salary
----+-------+-----+----------------------------------------------------+--------
1 | Paul | 32 | California | 20000
3 | Teddy | 23 | Norway | 20000
6 | Kim | 22 | South-Hall | 45000
7 | James | 24 | Houston | 10000
(4 行记录)
找出 AGE(年龄) 字段不为 25 或 27 或，32的数据：
runoobdb=# SELECT * FROM company WHERE age NOT IN(25,27,32);
id | name | age | address | salary
----+-------+-----+----------------------------------------------------+--------
3 | Teddy | 23 | Norway | 20000
6 | Kim | 22 | South-Hall | 45000
7 | James | 24 | Houston | 10000
(3 行记录)
--这里就没有了age=32的数据
--in不等于between
BETWEEN
列出AGE范围在25到32的数据：
runoobdb=# SELECT * FROM company WHERE age BETWEEN 25 AND 32;
id | name | age | address | salary
----+-------+-----+----------------------------------------------------+--------
1 | Paul | 32 | California | 20000
2 | Allen | 25 | Texas | 15000
4 | Mark | 25 | Rich-Mond | 65000
5 | David | 27 | Texas | 85000
(4 行记录)
子查询
查询薪资大于65000的人的年龄分布；
可以先查询出所有薪资大于65000的数据；然后将返回的数据给下一个语句处理，查询出相应的年龄；
runoobdb=# SELECT age FROM company
runoobdb-# WHERE EXISTS(SELECT AGE FROM company WHERE salary > 6500);
age
-----
32
25
23
25
27
22
24
(7 行记录)
查询出薪资大于65000的人，然后查询大于这些人年龄的数据；
runoobdb=# SELECT * FROM company WHERE age > (SELECT age FROM company WHERE salary > 65000);
id | name | age | address | salary
----+------+-----+----------------------------------------------------+--------
1 | Paul | 32 | California | 20000
(1 行记录)
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