MySQL加入WHERE和空值

Question

我目前正在努力（很容易，我認爲）的SQL問題，但我似乎無法弄清楚。

假設我有以下表格：

Persons 
+-------+----+ 
| name | id | 
+-------+----+ 
| Steve | 1 | 
| John | 2 | 
+-------+----+ 

Information 
+----------+----+-----------+---------------------------------------------------+ 
| type  | id | linked_id | info            | 
+----------+----+-----------+---------------------------------------------------+ 
| persons | 1 | 1   | Info about Steve         | 
| cars  | 2 | 1   | Info about a car, aka not stored in Persons table | 
+----------+----+-----------+---------------------------------------------------+ 

如果我想的個人表和信息（類型=人）的一個子集，我的查詢會是這樣的：

SELECT * 
FROM Persons 
LEFT JOIN Information ON Persons.id = Information.linked_id 
WHERE (Information.type = "persons" OR Information.type IS NULL) 

這應該是我所期望的：

Desired Result 
+-------+----+----------+------+------------+------------------+ 
| name | id | type  | id | linked_id | info    | 
+-------+----+----------+------+------------+------------------+ 
| Steve | 1 | persons | 1 | 1   | Info about Steve | 
| John | 2 | NULL  | NULL | NULL  | NULL    | 
+-------+----+----------+------+------------+------------------+ 

但這是實際結果：

+-------+----+----------+----+-----------+------------------+ 
| name | id | type  | id | linked_id | info    | 
+-------+----+----------+----+-----------+------------------+ 
| Steve | 1 | persons | 1 | 1   | Info about Steve | 
+-------+----+----------+----+-----------+------------------+ 

的「約翰」的人行，誰不尚未有一個信息行，也應包括在結果，但事實並非如此。

我在做什麼錯？我的查詢的OR Information.type IS NULL部分不應該照顧這個嗎？該行不包括在內。我錯過了別的嗎？

Answer 1

您需要將條件放在ON子句中，因爲它在加入表之前執行。

SELECT * 
FROM Persons 
     LEFT JOIN Information 
      ON Persons.id = Information.linked_id AND 
      Information.type = 'persons' 

• SQLFiddle Demo

輸出

╔═══════╦════╦═════════╦═══════════╦══════════════════╗ 
║ NAME ║ ID ║ TYPE ║ LINKED_ID ║  INFO  ║ 
╠═══════╬════╬═════════╬═══════════╬══════════════════╣ 
║ Steve ║ 1 ║ persons ║ 1   ║ Info about Steve ║ 
║ John ║ 2 ║ (null) ║ (null) ║ (null)   ║ 
╚═══════╩════╩═════════╩═══════════╩══════════════════╝