Question

我想使用MYSQL进行左连接。目前我有一个这样的表：

CREATE TABLE `books` (
  `bookId` int(11) NOT NULL,
  `bookTitle` varchar(100) NOT NULL,
  `bookAuthor` varchar(100) NOT NULL,
  `bookStatus` tinyint(1) NOT NULL,
  `bookDeleteFlag` tinyint(1) NOT NULL
);  
CREATE TABLE `lends` (
  `lendId` int(11) NOT NULL,Primary
  `lendBookId` int(11) NOT NULL,
  `lendBorrowerName` Varchar(100) NOT NULL,
  `lendBorrowStatus` int(11) NOT NULL,
  `lendReturnStatus` int(11) NOT NULL,
  );

insert into books values (1, 'The Da Vinci Code', 'Dan Brown', 1,0)
insert into books values (2, 'Theory of Relativity', 'Albert Einstein', 1,0)
insert into books values (3, 'Harry Potter', 'J K Rowling', 1,0)

insert into books values (1, '1', 'Chris', 1,1)
insert into books values (2, '1', 'Lilly', 1,0)
insert into books values (3, '2', 'Chris', 1,0)
insert into books values (3, '3', 'Chris', 1,1)

这样的期望输出

bookId         bookTitle           availability
-----------------------------------------------
  1        The Da Vinci Code            0
  2        Theory of Relativity         0
  3        Harry Potter                 1

我基本上正在开发一个图书馆管理模块。我希望该书的可用性列在图书搜索页面上。

我当前的代码是：

SELECT B.bookTitle,
       L.lendBorrowerName AS takenName,
       count(L.lendStatus) AS taken
FROM   books as B
LEFT JOIN lends AS L ON B.bookId = L.lendBookID
WHERE L.lendReturnStatus = 0 // if i remove this code, all rows in books table is listed. However i loose the ability to check the availability of that book
GROUP BY B.bookTitle

此问题的典型解决方案是什么？提前谢谢。

Answer 1

您需要将where子句中的条件移动到on子句。如果没有匹配的行，则该列的值为NULL且WHERE条件失败：

SELECT B.bookTitle,
       L.lendBorrowerName AS takenName,
       count(L.lendStatus) AS taken
FROM   books as B LEFT JOIN
       lends AS L
       ON B.bookId = L.lendBookID AND
          L.lendReturnStatus = 0
GROUP BY B.bookTitle;

Answer 2

SELECT B.bookTitle,
       sum(L.lendReturnStatus = 0) = 0 AS availibility
FROM   books as B
LEFT JOIN lends AS L ON B.bookId = L.lendBookID
GROUP BY B.bookTitle

带有条件的LEFT JOIN不会返回所需的数据

2 个答案: