MySQL-DML-condition query

• DQL conditional query

  • Condition query: select field list from table name where condition list

  • comparison operator

    • Equal operator (=)
      • specific code

      • -- 1.查询姓名为‘杨逍’的员工
        select id,
               username,
               password,
               name,
               gender,
               image,
               job,
               entrydate,
               create_time,
               update_time
        from tb_emp
        where name = '杨逍';

    • Not equal to operator (<> or !=)
      • The specific code is as follows

      • -- 5.查询密码不等于‘123456’的员工信息
        select id,
               username,
               password,
               name,
               gender,
               image,
               job,
               entrydate,
               create_time,
               update_time
        from tb_emp
        where password != '123456';

    • greater than operator (>)
    • less than operator (<)
    • greater than or equal to operator (>=)
    • less than or equal to operator (<=)
      • The specific code is as follows

      • -- 2.查询id小于等于5的员工信息
        select id,
               username,
               password,
               name,
               gender,
               image,
               job,
               entrydate,
               create_time,
               update_time
        from tb_emp
        where id <= 5;

    • BETWEEN.....AND....operator (including the most value)
      • The specific code is as follows

      • -- 6.查询入职日期在‘2000-0-01’（包含）到‘2010-01-01’（包含）之间的员工信息
        select id,
               username,
               password,
               name,
               gender,
               image,
               job,
               entrydate,
               create_time,
               update_time
        from tb_emp
        where entrydate between '2000-01-01' and '2010-01-01';

    • IN operator
      • The specific code is as follows

      • -- 8.查询职位是2（讲师）,3（学工主管），4（教研主管）的员工信息
        select id,
               username,
               password,
               name,
               gender,
               image,
               job,
               entrydate,
               create_time,
               update_time
        from tb_emp
        where job in (2, 3, 4);

    • The LIKE operator (placeholder) fuzzy matching uses like (_ matches a single character, % matches a character)
      • The specific code is as follows

      • -- 9.查询姓名为两个字的员工信息
        select id,
               username,
               password,
               name,
               gender,
               image,
               job,
               entrydate,
               create_time,
               update_time
        from tb_emp
        where name like '__';

    • IS NULL operator ( without =null )
      • The specific code is as follows

      • -- 3.查询没有分配职位的员工信息
        select id,
               username,
               password,
               name,
               gender,
               image,
               job,
               entrydate,
               create_time,
               update_time
        from tb_emp
        where job is null;

      • -- 10.查询性‘张’的员工信息
        select id,
               username,
               password,
               name,
               gender,
               image,
               job,
               entrydate,
               create_time,
               update_time
        from tb_emp
        where name like '张%';

    • IS NOT NULL operator ( and no !=null )
      • The specific code is as follows

      • -- 4.查询有职位的员工信息
        select id,
               username,
               password,
               name,
               gender,
               image,
               job,
               entrydate,
               create_time,
               update_time
        from tb_emp
        where job is not null;

  • Logical Operators

    • AND/&& operator: used to satisfy multiple conditions at the same time.
      • The specific code is as follows

      • -- 7.查询入职日期在‘2000-0-01’（包含）到‘2010-01-01’（包含）之间且性别为女的员工信息
        select id,
               username,
               password,
               name,
               gender,
               image,
               job,
               entrydate,
               create_time,
               update_time
        from tb_emp
        where entrydate between '2000-01-01' and '2010-01-01'
          and gender = 2;
        

    • OR/|| operator: used to meet any condition.
      • The specific code is as follows

      • -- 8.查询职位是2（讲师）,3（学工主管），4（教研主管）的员工信息
        select id,
               username,
               password,
               name,
               gender,
               image,
               job,
               entrydate,
               create_time,
               update_time
        from tb_emp
        where job = 2
           or 3
           or 4;

    • NOT/! Operator: Used to negate a condition.