多表连接查询的介绍
多表连接：
多表连接查询是从多个表中获取数据
语法结构：
table_reference
[INNER] JOIN |{LEFT|RIGHT }[OUTER] JOIN
table_reference
ON conditional_expr
eg:
– 多表连接
– 通过查看图书信息表和图书类别表来获取图书编号、图书名称、图书类别
SELECT book_id,book_name,category FROM bookinfo INNER JOIN bookcategory ON bookinfo.book_category_id=bookcategory.category_id;
内连接
什么是内连接
根据连接条件从多个表中查询选择数据，显示这些表中与连接条件相匹配的数据行，组合成新纪录。
SELECT column_list
FROM t1
[INNER] JOIN t2 ON join_condition1
[INNER] JOIN t3 ON join_condition2
…]
WHERE where_conditions;
案例：
SELECT borrowinfo.book_id,book_name,borrowinfo.card_id,NAME,tel,return_date,STATUS FROM borrowinfo
INNER JOIN bookinfo ON borrowinfo.book_id=bookinfo.book_id
INNER JOIN readerinfo ON borrowinfo.card_id=readerinfo.card_id
WHERE borrowinfo.status=‘否’;
起别名简化——
SELECT t1.book_id,book_name,t1.card_id,NAME,tel,return_date,STATUS FROM borrowinfo t1，INNER可以省略：
JOIN bookinfo t2 ON t1.book_id=t2.book_id
JOIN readerinfo t3 ON t1.card_id=t3.card_id
WHERE t1.status=‘否’;
外连接
外连接
外连接将查询多个表中相关联的行。
外连接分为：
左外连接LEFT[OUTER] JOIN
右外连接RIGHT[OUTER] JOIN
左外连接显示左表全部记录，右表满足连接条件的记录
右外连接显示右表全部记录，左表满足连接条件的记录
语法结构：
SELECT column_list
FROM t1
LEFT|RIGHT [OUTER] JOIN t2 ON jion_condition1;
案例：
1、左表连接
SELECT book_id,book_name,category FROM bookcategory
LEFT JOIN bookinfo ON bookcategory.category_id=bookinfo.book_category_id;
2、右表连接
SELECT book_id,book_name,category FROM bookinfo
RIGHT JOIN bookcategory ON bookcategory.category_id=bookinfo.book_category_id WHERE parent_id<>0;
起别名简化：
SELECT book_id,book_name,category FROM bookcategory b
LEFT JOIN bookinfo a ON b.category_id=a.book_category_id WHERE parent_id<>0;
自连接
如果在一个连接查询中，涉及的两个表都是同一个表，这种查询称为自连接查询。
自连接是一种特殊的连接查询，它是指相互连接的表在物理上为同一张表，但可以在逻辑上分为两张表。
案例：
查询所有图书类别的图书类别编号，类别名称，上级分类名称。
分析：
查看图书类别信息表
SELECT s.category_id AS ‘图书类别编号’,s.category AS ‘图书类别名称’,p.category AS ‘图书的上机分类名称’ FROM bookcategory s
LEFT JOIN bookcategory p
ON s.parent_id=p.category_id;
采用左连接或者自连接看具体需求。
多表更新
UPDATE
table1 {[INNER]JOIN|{LEFT|RIGHT}[OUTER]JOIN} table2
ON conditional_expr
SET col1={expr1|DEFAULT}
[col2={expr2|DEFAULT}]…
[WHERE where_condition]
案例：
UPDATE readerfee t1 JOIN readerinfo t2 ON t1.card_id=t2.card_id
SET actual_return_date=SYSDATE(),book_free=DATEDIFF(SYSDATE(),return_date)*0.2,balance=balance-book_free
WHERE t1.book_id=‘20190801’ AND t2.card_id=‘210210199901011111’;
多表删除
DELECT table1[.],table[.]
FROM table1{[INNER]JOIN|{LEFT|RIGHT}[OUTER]JOIN} table2
ON conditional_expr
[WHERE where_condition]
案例：
复制表
CREATE TABLE bookcategory_bak
AS
SELECT * FROM bookcategory;
CREATE TABLE bookinfo_bak
AS
SELECT * FROM bookinfo;
1、
DELETE t1 FROM bookcategory_bak t1
LEFT JOIN bookinfo_bak t2
ON t1.category_id=t2.book_category_id
WHERE parent_id<>0 AND book_id IS NULL;
2、
DELETE t1,t2 FROM bookcategory_bak t1
INNER JOIN bookinfo_bak t2
ON t1.category_id= t2.book_category_id
WHERE t1.category_id=3;
