PostgreSQL 中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。
语法
以下是 SELECT 语句中使用 WHERE 子句从数据库中读取数据的通用语法：
SELECT column1, column2, columnN
FROM table_name
WHERE [condition1]我们可以在 WHERE 子句中使用比较运算符或逻辑运算符，例如 >, <, =, LIKE, NOT 等等。
COMPANY.SQL文件内容如下：
-- This is the file to create COMPANY table and to populate it with 7 records.
-- Just copy and past them on psql prompt.
DROP TABLE COMPANY;
CREATE TABLE COMPANY(
ID INT PRIMARY KEY NOT NULL,
NAME TEXT NOT NULL,
AGE INT NOT NULL,
ADDRESS CHAR(50),
SALARY REAL
);
INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY)
VALUES (1, 'Paul', 32, 'California', 20000.00 );
INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY)
VALUES (2, 'Allen', 25, 'Texas', 15000.00 );
INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY)
VALUES (3, 'Teddy', 23, 'Norway', 20000.00 );
INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY)
VALUES (4, 'Mark', 25, 'Rich-Mond ', 65000.00 );
INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY)
VALUES (5, 'David', 27, 'Texas', 85000.00 );
INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY)
VALUES (6, 'Kim', 22, 'South-Hall', 45000.00 );
INSERT INTO COMPANY VALUES (7, 'James', 24, 'Houston', 10000.00 );创建COMPANY 表，数据内容如下：
sfjvipdb# select * from COMPANY;
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 rows)以下几个示例我们使用逻辑运算符来读取表中的数据。
AND
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：
sfjvipdb=# 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 rows)OR
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：
sfjvipdb=# 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 rows)NOT NULL
在公司表中找出 AGE(年龄) 字段不为空的记录：
sfjvipdb=# 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 rows)LIKE
在 COMPANY 表中找出 NAME(名字) 字段中以 Pa 开头的的数据：
sfjvipdb=# SELECT * FROM COMPANY WHERE NAME LIKE 'Pa%';
id | name | age |address | salary
----+------+-----+-----------+--------
1 | Paul | 32 | California| 20000IN
以下 SELECT 语句列出了 AGE(年龄) 字段为 25 或 27 的数据：
sfjvipdb=# 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 rows)NOT IN
以下 SELECT 语句列出了 AGE(年龄) 字段不为 25 或 27 的数据：
sfjvipdb=# 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 rows)BETWEEN
以下 SELECT 语句列出了 AGE(年龄) 字段在 25 到 27 的数据：
sfjvipdb=# SELECT * FROM COMPANY WHERE AGE BETWEEN 25 AND 27;
id | name | age | address | salary
----+-------+-----+------------+--------
2 | Allen | 25 | Texas | 15000
4 | Mark | 25 | Rich-Mond | 65000
5 | David | 27 | Texas | 85000
(3 rows)子查询
以下的 SELECT 语句使用了 SQL 的子查询，子查询语句中读取 SALARY(薪资) 字段大于 65000 的数据，然后通过 EXISTS 运算符判断它是否返回行，如果有返回行则读取所有的 AGE(年龄) 字段。
sfjvipdb=# SELECT AGE FROM COMPANY
WHERE EXISTS (SELECT AGE FROM COMPANY WHERE SALARY > 65000);
age
-----
32
25
23
25
27
22
24
(7 rows)以下的 SELECT 语句同样使用了 SQL 的子查询，子查询语句中读取 SALARY(薪资) 字段大于 65000 的 AGE(年龄) 字段数据，然后用 > 运算符查询大于该 AGE(年龄) 字段数据：
sfjvipdb=# SELECT * FROM COMPANY
WHERE AGE > (SELECT AGE FROM COMPANY WHERE SALARY > 65000);
id | name | age | address | salary
----+------+-----+------------+--------
1 | Paul | 32 | California | 20000