Sql左側使用filter連接兩列

Question

更新：這已解決，我正在進行語法錯誤。

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我可以加入並過濾左連接中的兩列嗎？ 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

我可以添加一個標志，以便我可以確定食物對是否已連接或是否為空？ 我不想內心加入，因為我需要計算每人的食物/食物總量，以及每人的食物/配對數量。 我試過了：

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。 基本上我只需要標記何時不應用連接但我遇到了麻煩。

Answer 1

我想你想要的是這個：

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

對於MySQL，表達式將返回布爾值true / false 1/0，在查找布爾輸出時用作簡寫CASE語句。

這將標記與1不匹配：

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

或者這將非匹配標記為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

嘗試在join之前標記null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

當表達式為NULL時返回1，否則返回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