表单查询

1.单表查询

SELECT colum_name1,colum_name2,...
	FROM table1_name
	[WHERE conditional_express1,conditional_express2,...]
	[GROUP BY conditional_express1,conditional_express2,...[ASC|DESC]]
	[LIMIT [OFFSET] data_num]

1.条件查询

where 后跟条件表达式，条件表达式是由字段名、关系运算符和数据，如

SELECT * FROM table1_name WHERE colum_name = data1;

带 in 表达式：

SELECT * FROM table1_name WHERE colum_name1 [NOT] IN (data1,data2,...);

带between and

SELECT * FROM table1_name WHERE colum_name BETWEEN data1 AND data2;

带 is null

SELECT * FROM table1_name WHERE colum_name IS NULL;

带 distinct【去重】

SELECT DISTINCT colum_name FROM table1_name; 

带like运算符【其中通配符 %代表任意多个字符，_代表任意一个字符，若字符里包含%和下划线可用\转义】

SELECT * FROM table1_name WHERE colum LIKE '%_';

带and或or查询

SELECT * FROM table1_name WHERE colum_name = data AND colum_name1 = data1;
SELECT * FROM table1_name WHERE colum_name =  data OR colum_name1 = data1;

2.聚合函数查询

用 min()、max()、avg()、count()、sum() 进行查询

SELECT def(colum_name) FROM table1_name;

3.排序查询【asc升序】

SELECT * FROM table1_name [WHERE] ORDER BY colum_name1,colum_name2,...[ASC/DESC];

4.分组查询

SELECT * FROM table1_name GROUP BY colum_name [HAVING def() = colum_data];

5.限制数量查询

SELECT * FROM table_name LIMIT [start_num,]colum_num;

2.多表查询

1.内连接

SELECT * FROM table1_name JOIN table2_name ON table1_name.colum_name = table2_name.colum_name;

2.左(右)连接

SELECT * FROM table1_name [LEFT/RIGHT]JOIN table2_name ON table1_name.colum_name = table2_name.colum_name;