MySQL-multi-table query-subquery (scalar, column)

subquery

• overview

  • Introduction: Nested select statements in SQL statements are called nested queries, also known as subqueries
  • 形式：select * from t1 where column1 = （select column1 from t2.....）
  • The subquery external statement can be any one of insert/update/delete/select, the most common is select

• Classification

  • scalar subqueries

    • The result returned by a subquery is a single value (number, string, date, etc.), in its simplest form
      •  
    • Common symbols: =, <>, >, >=, <, <=
    • The specific demo code is as follows

      • -- todo 子查询
        -- 标量子查询
        -- A.查询“教研部”的所有员工信息
        -- a.查询 教研部 的部门id
        select tb_dept.id
        from tb_dept
        where name = '教研部';
        -- b.再查询该部门id下的员工信息
        select *
        from tb_emp
        where dept_id = 2;
        -- 最终语句如下
        select *
        from tb_emp
        where dept_id = (select id from tb_dept where name = '教研部');
        -- select id from tb_dept where name = '教研部'   该语句属于子查询
        
        -- B.查询在“东方白”入职后的员工信息
        -- a.查询’东方白‘的入职时间
        select entrydate
        from tb_emp
        where name = '方东白';
        -- b.查询在方东白入职之后的员工信息
        select *
        from tb_emp
        where entrydate > '2012-11-01';
        -- 最终语句如下
        select *
        from tb_emp
        where entrydate > (select entrydate
                           from tb_emp
                           where name = '方东白')
        -- 子查询的语句
        # select entrydate
        # from tb_emp
        # where name = '方东白'

  • column query

    • The result returned by the subquery is a column (can be multiple rows),

      •  

    • Commonly used symbols: in, not in, etc.
    • The specific demo code is as follows

      • -- todo 列子查询
        -- A.查询’教研部‘和’咨询部‘员工的所有信息
        -- a.获取两部门的部门id
        select id
        from tb_dept
        where name = '教研部'
           or name = '咨询部';
        -- b.根据部门id，查询该部门下的员工id
        select *
        from tb_emp
        where dept_id in (2, 3);
        -- 合并后的查询语句
        select *
        from tb_emp
        where dept_id in (select id
                          from tb_dept
                          where name = '教研部'
                             or name = '咨询部');

  • row subquery

    • The result returned by the subquery is one row (can be multiple columns)

      • 

      • The specific demo code is as follows

        • -- todo 行查询
          -- A.查询与‘韦一笑’的入职日期及职位都相同的员工信息
          -- a.查询‘韦一笑’ 的入职日期 及 职位
          select entrydate, job
          from tb_emp
          where name = '韦一笑';
          -- b.查询与‘韦一笑’的入职日期及职位相同的员工信息
          select *
          from tb_emp
          where entrydate = '2007-01-01'
            and job = 2;
          
          select *
          from tb_emp
          where (entrydate, job) = ('2007-01-01', 2);
          
          -- 合并查询语句
          select *
          from tb_emp
          where (entrydate, job) = (select entrydate, job
                                    from tb_emp
                                    where name = '韦一笑');

  • table subquery

    • The result returned by the subquery is multi-row and multi-column, often used as a temporary table

      • 

      • Commonly used operators: in

        •  Specific operation code

        • -- todo 表子查询
          -- A.查询入职日期是’2006-01-01‘之后的员工信息,及部门名称
          -- a.查询入职日期为’2006-01-01‘之后的员工信息
          select *
          from tb_emp
          where entrydate > '2006-01-01';
          -- b.查询这部分员工信息及其部门名称
          select *
          from (select *
                from tb_emp
                where entrydate > '2006-01-01') e,
               tb_dept d
          where e.dept_id = d.id;
          # select *
          # from tb_emp
          # where entrydate > '2006-01-01' 为子查询