1.简介
联合查询是多表查询的一种方式，经常应用在分表操作中，在保证多个SELECT语句的查询字段数相同的情况下，合并多个查询的结果。
语法
SELECT …
UNION [ALL | DISTINCT] SELECT …
[UNION [ALL | DISTINCT] SELECT …];
UNION是实现联合查询的关键字。
ALL表示保存所有的查询结果。
DISTINCT是默认值，可以省略，表示去除完全重复的记录
2.准备
CREATE DATABASE mahaiwuji;
USE mahaiwuji;
CREATE TABLE student1 (
sid INT (4) PRIMARY KEY,
sname VARCHAR (36),
score INT
) ENGINE = INNODB DEFAULT CHARSET = utf8;
INSERT INTO student1 VALUES (1,'a1',60);
INSERT INTO student1 VALUES (2,'a2',65);
INSERT INTO student1 VALUES (3,'a3',70);
INSERT INTO student1 VALUES (4,'a4',75);
INSERT INTO student1 VALUES (5,'a5',80);
CREATE TABLE student2 (
sid INT (4) PRIMARY KEY,
sname VARCHAR (36),
score INT
) ENGINE = INNODB DEFAULT CHARSET = utf8;
INSERT INTO student2 VALUES (5,'a5',80);
INSERT INTO student2 VALUES (6,'a6',85);
INSERT INTO student2 VALUES (7,'a7',90);
INSERT INTO student2 VALUES (8,'a8',95);
INSERT INTO student2 VALUES (9,'a9',100);
s24_1.png
s24_2.png
3.案例
-- 自动去掉完全重复的数据
SELECT * FROM student1
UNION
SELECT * FROM student2;
s24_3.png
-- 合并所有的数据
SELECT * FROM student1
UNION ALL
SELECT * FROM student2;
s22_4.png
-- 合并部分数据
SELECT * FROM student1 WHERE sid=1
UNION ALL
SELECT * FROM student2;
s24_5.png
-- 排序
SELECT * FROM student1
UNION ALL
SELECT * FROM student2
ORDER BY sid DESC;
s24_6.png
