Postgres——查詢判斷條件等於時可能包含空值的情況處理

查詢判斷條件等於時可能包含空值的情況處理

就是where后的條件語句，可能有空值null，也可能有正確的內容

目錄

• 查詢判斷條件等於時可能包含空值的情況處理
  • 場景
  • 思路
    • 錯誤的操作
  • 借鑒
  • 結果

場景

被這個需求搞得很頭大，正常情況下，關聯表查詢，我是這么寫的

SELECT 
  mp.xx, other_fields
FROM
  cargo C
  LEFT JOIN market_price mp ON c.product_id = mp.product_id and c.level_id = mp.level_id and c.brand_id = mp.brand_id

但是mp表的數據不是全部都有值，有的是空
然而數據庫對於空值null，進行操作都會返回null，所以就可以理解為這個關聯查詢的條件不生效，關聯查詢拿不到一條結果
= 一個值，但是這個值有可能為null，就不生效的問題

思路

錯誤的操作

1. 我嘗試了使用如下sql代碼

SELECT 
  mp.xx, other_fields
FROM
  cargo C
  LEFT JOIN market_price mp ON ( ( C.brand_id IS NOT NULL AND C.brand_id = mp.brand_id ) OR C.brand_id IS NULL ) 
	AND ( ( C.product_id IS NOT NULL AND C.product_id = mp.product_id ) OR C.product_id IS NULL ) 
	AND ( ( C.level_id IS NOT NULL AND C.level_id = mp.level_id ) OR C.level_id IS NULL ) 

雖然拿到了數據，但是后知后覺提交了代碼測試的時候發現不對勁，存在重復的數據，GG
2. 我又嘗試在where中使用case when xx then result else other_result end
但很遺憾，數據依然有重復的

借鑒

最后在我絕望地百度了一堆帖，基本都是在講解COALESCE如何將查詢到的結果轉換成另一個值，但是這並不符合我的情況，發現了如下一段很不起眼的內容
PostgreSQL WHERE和null

我將希望寄托在IS NOT DISTINCT FROM語句上
果然我就找到了解決的辦法！
postgresql 的 is distinct from、is not distinct from
如下為摘抄內容：

• is distinct from
  功能描述
  A和B的數據類型、值不完全相同返回 true
  A和B的數據類型、值完全相同返回 false
  將空值視為相同。

postgres=# \x
Expanded display is on.
postgres=# select 1 is distinct from 1,
1 is distinct from 2,
1 is distinct from '1',
'1' is distinct from '1',
1 is distinct from null,
null is distinct from null
;
-[ RECORD 1 ]
?column? | f
?column? | t
?column? | f
?column? | f
?column? | t
?column? | f

• is not distinct from
  功能描述
  A和B的數據類型、值不完全相同返回 false
  A和B的數據類型、值完全相同返回 true
  將空值視為相同。

postgres=# \x
Expanded display is on.
postgres=# select 1 is not distinct from 1,
1 is not distinct from 2,
1 is not distinct from '1',
'1' is not distinct from '1',
1 is not distinct from null,
null is not distinct from null
;
-[ RECORD 1 ]
?column? | t
?column? | f
?column? | t
?column? | t
?column? | f
?column? | t

拋開=和is符號的限制，進行判斷相等，這不就是我苦苦尋覓的滑板鞋嗎！？！？

結果

SELECT 
  mp.xx, other_fields
FROM
  cargo C
  LEFT JOIN market_price mp ON
--  (前 與 后 相等為 true)  = true
    (c.product_id is not DISTINCT from mp.product_id) = true and
    (c.level_id is not DISTINCT from mp.level_id) = true and
    (c.brand_id is not DISTINCT from mp.brand_id) = true

Nice 兄die~