INNER JOIN(内连接):取得两个表中存在连接匹配关系的记录
$sql="SELECT * FROM subject as a INNER JOIN e_user as b ON a.uid=b.id"; //也可以用以下方法 SELECT article.aid,article.title,user.username FROM article,user WHERE article.uid = user.uid
LEFT JOIN (左外连):会取得左表(table1)全部记录,即使右表(table2)并无对应匹配记录
$sql="SELECT * FROM subject as a LEFT JOIN e_user as b ON a.uid=b.id";
在上面的例子中,对于右表中没有对应匹配的数据记录,其所有的列都被置为 NULL,因此要查询这部分记录(如在上面例子中体现为查找 id=4 这类无对应用户的记录),可以附加 IS NULL 条件:
$sql="SELECT * FROM subject as a LEFT JOIN e_user as b ON a.uid=b.id WHERE b.id IS NULL";
RIGHT JOIN(右外连): 会取得右表(table2)全部记录,即使左表(table2)并无对应匹配记录
$sql="SELECT * FROM subject as a RIGHT JOIN e_user as b ON a.uid=b.id";
除了常用的两个表连接之外,SQL(MySQL) JOIN 语法还支持多表连接
SELECT article.aid,article.title,user.username,type.typename FROM article INNER JOIN user ON article.uid=user.uid INNER JOIN type ON article.tid=type.tid
//LEFT JOIN
SELECT article.aid,article.title,user.username,type.typename FROM article LEFT JOIN user ON article.uid=user.uid LEFT JOIN type ON article.tid=type.tid
//RIGHT JOIN
SELECT article.aid,article.title,user.username,type.typename FROM article RIGHT JOIN user ON article.uid=user.uid RIGHT JOIN type ON article.tid=type.tid
子查询
子查询是将一个 SELECT 语句的查询结果作为中间结果,供另一个 SQL 语句调用
$sql = "SELECT * FROM e_user WHERE id IN (SELECT uid FROM subject WHERE fun>80)"; //等同于 SELECT * FROM e_user WHERE id IN(1,2)
标量子查询
$sql = "SELECT * FROM e_user WHERE id=(SELECT min(uid) FROM subject)";
WHERE 表达式中运算符说明:
运算符 | 说明 |
---|---|
= | 等于 |
!= | 不等于,某些数据库系统也写作 <> |
> | 大于 |
< | 小于 |
>= | 大于或等于 |
<= | 小于或等于 |
BETWEEN … AND … | 介于某个范围之内,例:WHERE age BETWEEN 20 AND 30 |
NOT BETWEEN …AND … | 不在某个范围之内 |
IN(项1,项2,…) | 在指定项内,例:WHERE city IN('beijing','shanghai') |
NOT IN(项1,项2,…) | 不在指定项内 |
LIKE | 搜索匹配,常与模式匹配符配合使用 |
NOT LIKE | LIKE的反义 |
IS NULL | 空值判断符 |
IS NOT NULL | 非空判断符 |
NOT、AND、OR | 逻辑运算符,分别表示否、并且、或,用于多个逻辑连接。 优先级:NOT > AND > OR |
% | 模式匹配符,表示任意字串,例:WHERE username LIKE '%user' |
查询之后插入到另一个表
INSERT INTO other(id,sid,name,age,subject,total) SELECT null,s.id, s.name,s.age,j.subject,j.total from SUBJECT as j LEFT JOIN student as s on s.id=j.sid