多表操作-UNION ALL 合并讲解

#UNION,UNION ALL 合并结果使用
-- 1.如果使用UNION,操作的表必须要有相同的列，并且每个列类型都是一样
CREATE TABLE employee_china(
    id int,
    name varchar(50)
);
CREATE TABLE employee_usa(
    id int,
    name varchar(50)
);

INSERT INTO employee_usa VALUES (1,'michal');
INSERT INTO employee_usa VALUES (2,'lucy');
INSERT INTO employee_usa VALUES (3,'anmy');

INSERT INTO employee_china VALUES (1,'永马云');
INSERT INTO employee_china VALUES (2,'郭峰');
INSERT INTO employee_china VALUES (3,'马化腾');

INSERT INTO employee_usa VALUES (4,'vincent');
INSERT INTO employee_china VALUES (4,'vincent');

SELECT * FROM employee_china;
SELECT * FROM employee_usa;

#合并两张表
-- UNION 去除重复
SELECT * FROM employee_china UNION SELECT * FROM employee_usa; --结果集为7条
-- UNION ALL 不去除重复
SELECT * FROM employee_china UNION ALL SELECT * FROM employee_usa；--结果集为8条