Embedded database development programming (V) -- DQL

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

1. select distinct name from students;//4 Query students with different names ;
2. 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