MySQL中union和unionall差別是什麼

union:對多個結果集進行並集操作，不包括重複行，同時進行排序。

union all:對多個結果集進行並集操作，包括重複行，不進行排序。

查詢部門小於30號的員工訊息，和部門大於20小於40號的員工資訊。

①.先查詢部門小於30號的員工資料。

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

查詢部門大於20小於40的員工資料。

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

③.用union連接兩張表

    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; ```

其結果預設排序並去重，兩張表都有30號部門信息，結果只出現一次。

④.下面用union all連接兩張表

     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; ```

其結果沒有去重，也沒有排序，排序結果對比下邊結果，先去查詢20到40的員工信息，在查小於30的員工資訊。

⑤.比較查詢結果

     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;```

預設是沒有進行排序的。

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