本文主要是介绍一张图告诉你常用的SQL连接，内连接、左外连接、右外连接、全连接，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
橙红色为结果集
附SQL语句：
-- 创建表并插入数据
CREATE TABLE person(id VARCHAR(10),NAME VARCHAR(20),parentid VARCHAR(10));
INSERT INTO person(id,NAME,parentid) VALUES('1','李四','1');
INSERT INTO person(id,NAME,parentid) VALUES('2','王五','1');
INSERT INTO person(id,NAME,parentid) VALUES('3','赵六','4');
SELECT * FROM person;
SELECT * FROM place;
CREATE TABLE place(id VARCHAR(10),NAME VARCHAR(20));
INSERT INTO place(id,NAME) VALUES('1','供销社');
INSERT INTO place(id,NAME) VALUES('2','服务部');
INSERT INTO place(id,NAME) VALUES('3','超市');
-- 内连接
SELECT * FROM person INNER JOIN place ON person.`parentid` = place.`id`;
-- 左外连接
SELECT * FROM person LEFT JOIN place ON person.`parentid` = place.`id`;
-- 左外连接代替内连接
SELECT * FROM person LEFT JOIN place ON person.`parentid` = place.`id` WHERE place.`id` IS NOT NULL;
-- Union代替全连接
SELECT * FROM person LEFT JOIN place ON person.`parentid` = place.`id` UNION
SELECT * FROM person RIGHT JOIN place ON person.`parentid` = place.`id`;
-- SELECT * FROM person FULL JOIN place ON person.`parentid` = place.`id`;
SELECT * FROM person,place;
PS：
1、MySQL不支持全连接，所以我们通过SELECT * FROM person LEFT JOIN place ON person.`parentid` = place.`id` UNION
SELECT * FROM person RIGHT JOIN place ON person.`parentid` = place.`id`;来代替
2、考虑性能，我们用左外连接来代替内连接
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