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Embedded database development programming (V) -- DQL
2022-07-05 05:07:00 【Light chasing rain】
List of articles
One 、 Data sets
Two 、 Query log
select field1,field2,…fieldn… from Table name [where Conditions ]
select*from Table name [where Conditions ]
3、 ... and 、 Query non duplicate records
- select distinct Field from Table name
- select distinct name from students;//4 Query students with different names ;
- select distinct name,age from students;// Query students with different names and ages
- distinct Must be placed at the beginning
- distinct You can only use fields that need to be de duplicated
- distinct When removing multiple fields , The meaning is : Only when several fields are repeated at the same time can they be filtered .
Four 、 Conditions of the query
select Field from Table name where Conditions ; select*from student where sex=' male ’and age>20; // Query gender is male , And older than 20
where The following conditions can be used >、<、>=、<=、!= And other comparison operators , Between multiple conditions, you can use or、and And so on
5、 ... and 、 Sort and limit
5.1 Sort
5.2 Limit
6、 ... and 、 polymerization
6.1 Aggregate functions
7、 ... and 、 Multi-table query
8、 ... and 、 Table joins
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