18 关于查询结果集的去重？

18 关于查询结果集的去重？

    select distinct job from emp; // distinct 关键字 去除重复记录。

        +-----------+

        | job       |

        +-----------+

        | CLERK     |

        | SALESMAN  |

        | MANAGER   |

        | ANALYST   |

        | PRESIDENT |

        +-----------+

 

    select ename,distinct job from emp;

    这条语句是错误的。

    记住：distinct只能出现在所有字段的最前面

    

    select distinct deptno,job from emp;

        +--------+-----------+

        | deptno | job       |

        +--------+-----------+

        |     20 | CLERK     |

        |     30 | SALESMAN  |

        |     20 | MANAGER   |

        |     30 | MANAGER   |

        |     10 | MANAGER   |

        |     20 | ANALYST   |

        |     10 | PRESIDENT |

        |     30 | CLERK     |

        |     10 | CLERK     |

        +--------+-----------+

    

案例:统计岗位的数量?

    select count(distinct job) from emp;

        +---------------------+

        | count(distinct job) |

        +---------------------+

        |                   5 |

        +---------------------+