union和unionall区别

科技前沿 • 2025-03-10 09:24 • 阅读 38

大家好，我是讯享网，很高兴认识大家。

查询部门小于30号的员工信息，和部门大于20小于40号的员工信息。

①.先查询部门小于30号的员工信息。

 SELECT employees_id ,last_name ,salary ,department_id FROM employees WHERE department_id < 30; ```

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小于30号部门的员工信息
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查询部门大于20小于40的员工信息。

讯享网 SELECT employees_id ,last_name ,salary ,department_id FROM employees WHERE department_id BETWEEN 20 and 40;```

20到40号部门的员工信息

③.用union连接两张表

 SELECT employees_id ,last_name ,salary ,department_id FROM employees WHERE department_id < 30 UNION SELECT employees_id ,last_name ,salary ,department_id FROM employees WHERE department_id BETWEEN 20 and 40; ```

其结果默认排序并去重，两张表都有30号部门信息，结果只出现一次。

union连接两张表

④.下面用union all连接两张表

讯享网 SELECT employees_id ,last_name ,salary ,department_id FROM employees WHERE department_id < 30 UNION ALL SELECT employees_id ,last_name ,salary ,department_id FROM employees WHERE department_id BETWEEN 20 and 40; ```

其结果没有去重，也没有排序，排序结果对比下边结果，先去查询20到40的员工信息，在查小于30的员工信息。
union all连接两张表

⑤.对比查询结果

 SELECT employees_id ,last_name ,salary ,department_id FROM employees WHERE department_id BETWEEN 20 and 40 UNION ALL SELECT employees_id ,last_name ,salary ,department_id FROM employees WHERE department_id < 30;```

union all去重对比

默认是没有进行排序的。