DML的数据查询指对数据库中的数据查询、统计、分组、排序等操作。查询语句可以分为简单查询、连接查询、嵌套查询和组合查询。-The DML data query refers to the data in the query, statistic, grouping, sorting, and so on. Query can be divided into simple query, join queries, nested query and combination query.