Question

我刚刚将我的网站上传到Heroku但是我的一个控制器出错了。显然我的postgres代码有语法错误。这适用于sqlite3但是当我将其转换为postgres时发生了语法错误。

   @cars=Car.find_by_sql("SELECT cars.*, CASE WHEN EXISTS (SELECT book_cars.* FROM book_cars WHERE book_cars.car_id=cars.id AND book='t') THEN 'TRUE' ELSE 'FALSE' END  AS 'is_book' FROM cars WHERE is_book='FALSE'")

有关导致错误的原因的任何想法？错误代码：

PG::SyntaxError: ERROR:  syntax error at or near "'is_book'"

Answer 1

尝试简单：

SELECT *, 'FALSE' As is_book
FROM cars c
WHERE NOT EXISTS (
   SELECT * FROM book_cars b
   WHERE b.car_id=c.id AND book = 't'
)

您也可以使用子查询：

SELECT *
FROM (
   SELECT cars.*, 
          CASE WHEN EXISTS (
              SELECT book_cars.* FROM book_cars 
              WHERE book_cars.car_id=cars.id AND book='t') 
          THEN 'TRUE' ELSE 'FALSE' END  AS is_book
   FROM cars 
) x
WHERE is_book='FALSE';

修改

您的查询中的问题位于以下最后一行的WHERE子句中：

SELECT cars.*, CASE WHEN EXISTS ( SELECT book_cars.* FROM book_cars WHERE book_cars.car_id=cars.id AND book='t') THEN 'TRUE' ELSE 'FALSE' END AS 'is_book' FROM cars WHERE is_book='FALSE';

在标准SQL WHERE子句中，没有看到SELECT子句中声明的任何表达式（或＆＃34;列＆＃34;），它只能看到表中的列FROM条款 is_book子句中声明了SELECT列，因此WHERE无法看到它如果使用子查询，例如：

SELECT * FROM ( --- subquery SELECT some_expression AS new_column FROM .... ) x WHERE new_column = 111

然后外部查询会看到新列。

您也可以使用HAVING子句：

SELECT cars.*, CASE WHEN EXISTS ( SELECT book_cars.* FROM book_cars WHERE book_cars.car_id=cars.id AND book='t') THEN 'TRUE' ELSE 'FALSE' END AS 'is_book' FROM cars HAVING is_book='FALSE';

因为与HAVING相比，WHERE确实会看到在SELECT级别声明的列。

将sqlite3查询转换为postgres

1 个答案: