MySQL-DQL-Basic Query

Database operation DQL statement

• DQL

  • DQL English full name Data Query Language (data query language), used to query the records in the database table
  • Keywords: SELECT
  • grammar
    • basic query
    • conditional query ( where )
    • Group query ( group by )
    • sort query ( order by )
    • Pagination query ( limit )

  • DQL - basic query

    • Query multiple fields: select field 1, field 2, field 3 from table name;

      • Specific code and results

      • -- 1.在查询指定字段 name entryDate 并返回
        select name, entrydate
        from tb_emp;

      • 

    • Query all fields (wildcard *): select *from table name

      • Specific code and results

      • -- 2.查询返回所有字段
        # select *
        # from tb_emp; -- 在具体项目中不推荐使用（不直观且性能低）
        select id,
               username,
               password,
               name,
               gender,
               image,
               job,
               entrydate,
               create_time,
               update_time
        from tb_emp;

      • 

    • Set the alias: select field 1 [ as alias 1], field 2 [ as alias 2] from table name;

      • Specific code and results

      • -- 3.查询所有员工的 name entrydate，并起别名（姓名、入职日期）
        # select name as '姓名', entrydate as '入职日期'
        # from tb_emp;
        
        select name '姓名', entrydate '入职日期'
        from tb_emp;

      • 

    • Remove duplicate records: select distinct field list from table name

      • Specific code and results

      • -- 4.查询已有员工关联了哪几种缘分（不要重复）
        select distinct job
        from tb_emp;

      •