php - 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。目前我有一个类似这样的表:

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)

想要的输出是这样的

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。我希望在图书搜索页面上列出该书的可用性。

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？ 提前致谢。

Answer 1

您需要将 where 子句中的条件移至 on 子句。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;