SQL 使用过滤器在两列之间左连接

Question

更新：这已解决，我犯了一个语法错误。

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是否可以在左连接中联接和筛选两列？ 例如：

tbl_people
id    food     side     value
a     pizza    fries    10
b     pizza    shake     2
c     burger   fries    3
tbl_sides
food     side
pizza    fries
burger   fries

然后使用 SQL：

SELECT 
  id, food, side, value
FROM
  tbl_people AS people
LEFT JOIN
  tbl_sides AS sides ON sides.food = people.food
  AND sides.side = people.side

我可以

添加一个标志，以便我可以确定食物对是否联接或是否为 NULL？ 我不想内部连接，因为我需要计算每人的食物/配菜总数，以及每人匹配的食物/配菜对。 我试过了：

SELECT 
  id, food, side, value,
  CASE WHEN
    side.side IS NOT NULL
    AND side.food IS NOT NULL
    THEN 1
    ELSE 0
  END AS match_flag
FROM
  tbl_people AS people
LEFT JOIN
  tbl_sides AS sides ON sides.food = people.food
  AND sides.side = people.side

但它不起作用。 基本上，我只需要在未应用连接时进行标记，但我遇到了麻烦。

Answer 1

我想你想要的是这个：

SELECT 
  id, food, side, value,
  CASE WHEN
    side.side = people.side 
    THEN 1
    ELSE 0
  END AS match_flag
FROM
  tbl_people AS people
LEFT JOIN
  tbl_sides AS sides ON people.food = sides.food

Answer 2

使用 MySQL，表达式将返回布尔值 true/false 1/0，在查找布尔输出时用作速记CASE语句。

这将用于标记与 1 不匹配的情况：

SELECT 
  people.id, people.food, people.side, people.value
  ,sides.food IS NULL AS match_flag
FROM
  tbl_people AS people
LEFT JOIN tbl_sides AS sides 
  ON sides.food = people.food
  AND sides.side = people.side

演示：SQL 小提琴

或者这样做将不匹配标记为 0：

SELECT 
  people.id, people.food, people.side, people.value
  ,COALESCE(sides.food = people.food,0) AS match_flag
FROM
  tbl_people AS people
LEFT JOIN tbl_sides AS sides 
  ON sides.food = people.food
  AND sides.side = people.side

演示：SQL 小提琴

Answer 3

尝试在连接之前标记空值

SELECT 
  people.id, people.food, people.side, people.value, nvl(sides.side, 'NOT JOINED')
FROM
  tbl_people AS people
LEFT JOIN
  (select food, nvl(side, 'SOME NULL') as side from tbl_sides) AS sides 
 ON sides.food = people.food
AND sides.side = people.side

Answer 4

当表达式为 NULL 时，它返回 1，否则返回 0。

http://sqlfiddle.com/#!2/cadbdd/15/0

SELECT *, ISNULL(sides.food) AS match_flag
FROM
  tbl_people AS people
LEFT JOIN
  tbl_sides AS sides ON sides.food = people.food
  AND sides.side = people.side