带条件的 LEFT JOIN 未返回所需的数据

Question

我想使用 MYSQL 进行左连接。目前我有一个 table 像这样的东西：

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)

想要的输出像这样

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

我基本上是在开发图书馆管理模块。我希望那本书的可用性列在图书搜索页面上。

我目前的代码是：

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

这个问题的典型解决方案是什么？提前致谢。

Answer 1

您需要将 where 子句中的条件移动到 on 子句中。当没有行匹配时，该列的值为 NULL，并且 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

带条件的 LEFT JOIN 未返回所需的数据

LEFT JOIN with condition is not returning data desired

php

mysql

join

left-join