检查连接表

Question

我有两张表：categories和prodvscats

categories              prodvscats

  id            |       id      
  title         |       categories_id

每个表都有一些行，例如：

categories              prodvscats

  categ1            |       categ1      
  categ2            |       categ2
  categ3
  categ4

我想返回所有类别，但我想知道prodvscats表中存在哪些类别。

期望的结果：

  title                boolean variable exists
  categ1            |       1      
  categ2            |       1
  categ3            |       0
  categ4            |       0

我尝试使用join，但它只返回前两行。也许它需要像个案一样，但我不知道该怎么做。

SELECT categories.id, categories.title
FROM categories
        INNER JOIN prodvscats ON prodvscats.categories_id = categories.id
ORDER BY id

Answer 1

使用LEFT JOIN加入这两个表。这将返回所有行，包括其他表中不存在类别的行。然后使用GROUP BY合并类别和COUNT以确定是否存在匹配项：

SELECT
    categories.id,
    categories.title,
    CASE WHEN COUNT(prodvscats.id) = 0 THEN 0 ELSE 1 END AS product_exists
FROM categories
LEFT JOIN prodvscats ON categories.id = prodvscats.categories_id 
GROUP BY categories.id, categories.title

Answer 2

试试这个

SELECT categories.id, categories.title, 
IF(prodvscats.categories_id IS NULL, '0', '1') AS exists_value
FROM categories
LEFT JOIN prodvscats ON prodvscats.categories_id = categories.id
ORDER BY id