mysql的学习笔记(五)

2022-01-18 16:39:21

1.子查询，出现在其他SQL语句的SELECT子句

SELECT * FROM t1 WHERE col1=(SELECT col2 FROM t2);

第一个SELECT称为外层查询，第二个称为子查询，嵌套在内部的查询。称为子查询。可以包含多个关键字或条件

DISTINCT,GROUP BY.ORDER BY,LIMIT，函数等。子查询可以返回值可以是标量，一行，一列或子查询。

外层查询可以是SELECT,INSERT,UPDATE,SET或DO

(1).比较运算符的子查询。

=,>,<,>=,<=,<>,!=,<=>

SELECT * FROM t1 WHERE price>=(SELECT AVG(price) t1 )

(2).ANY SOME ALL子查询在返回多个结果时可用修饰

（3).IN的子查询

SELECT * FROM t1 WHERE price IN (SELECT * FROM t1 WHERE price>5000)

2.INSET.....SELECT插入

INSET t1(cate_names) SELECT goods_cate FROM tdb_goods_cate GROUP BY goods_cate

3.多表的更新

UPDATE table_references SET col_name1={expr1|DEFAULT} [，col_name2={expr2|DEFAULT}]..[WHERE where_condition]

UPDATE tdb_goods INNER JOIN tdb_goods_cates ON goods_cate= cate_name SET goods_cate=cate_id;

4.多表更新一步到位

CREATE TABLE tdb_goods_brands(
brand_id SMALLINT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
brand_name VARCHAR(40) NOT NULL
)
SELECT brand_name FROM tdb_goods GROUP BY brand_name;

5.连接，mysql在SELECT语句，多表更新，多表删除语句中支持JOIN操作。有相同字段的名字，起别名

(1).内连接 INNER JOIN。仅显示符合连接的记录。

使用ON关键字来设定连接条件，也可用WHERE来代替

SELECT goods_id,goods_name FROM tdb_goods t INNER JOIN tdb_goods_cate g ON t.cate_id= g.cate_id

(2).左外连接，显示左边的全部记录及右表符合连接条件的记录

SELECT goods_id,goods_name FROM tdb_goods t LEFT JOIN tdb_goods_cate g ON t.cate_id= g.cate_id

(3).右外连接，显示右表的全部记录及左表符合连接条件的记录

SELECT goods_id,goods_name FROM tdb_goods t RIGHT JOIN tdb_goods_cate g ON t.cate_id= g.cate_id

6.多表连接

SELECT goods_id,goods_name,cate_name,brand_name FROM tdb_goods g INNER JOIN tdb_goods_cates c ON g.cate_id=c. cate_id INNER JOIN tdb_goods_brands b ON g.brand_id=b.brand_id

7.自连接，顾名思义自己连接自己

数据准备

  INSERT tdb_goods_types(type_name,parent_id) VALUES('家用电器',DEFAULT);
  INSERT tdb_goods_types(type_name,parent_id) VALUES('电脑、办公',DEFAULT);
  INSERT tdb_goods_types(type_name,parent_id) VALUES('大家电',1);
  INSERT tdb_goods_types(type_name,parent_id) VALUES('生活电器',1);
  INSERT tdb_goods_types(type_name,parent_id) VALUES('平板电视',3);
  INSERT tdb_goods_types(type_name,parent_id) VALUES('空调',3);
  INSERT tdb_goods_types(type_name,parent_id) VALUES('电风扇',4);
  INSERT tdb_goods_types(type_name,parent_id) VALUES('饮水机',4);
  INSERT tdb_goods_types(type_name,parent_id) VALUES('电脑整机',2);
  INSERT tdb_goods_types(type_name,parent_id) VALUES('电脑配件',2);
  INSERT tdb_goods_types(type_name,parent_id) VALUES('笔记本',9);
  INSERT tdb_goods_types(type_name,parent_id) VALUES('超级本',9);
  INSERT tdb_goods_types(type_name,parent_id) VALUES('游戏本',9);
  INSERT tdb_goods_types(type_name,parent_id) VALUES('CPU',10);
  INSERT tdb_goods_types(type_name,parent_id) VALUES('主机',10);

SELECT s.type_id,s.type_name,t.type_name FROM tdb_goods_types s LEFT JOIN tdb_goods_types t ON s.parent_id=t.type_id;

8.多表删除

DELETE t1 FROM tdb_goods t1 LEFT JOIN (SELECT goods_id,goods_name FROM tdb_goods GROUP BY goods_name HAVING count(goods_name)>=2) t2 ON t1.goods_name=t2.goods_name WHERE t1.goods_id>t2.goods_id

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