用postgresql中的左連接更新

Question

如何編寫查詢以更新具有兩個左連接的表。下面是MSSQL中的一個查詢，我想將它轉換爲postgresql。請幫忙。

Update T1 Set 
      T1.Amount = (T1.Amount - T2.Disc) + ((DT1.N_Amount)/2) 

    From @Before T1 
    Left Join @Before T2 On T1.ID = T2.ID 
    Left Join @DiscTable DT1 On T1.PurchID = DT1.Purch_ID 

Answer 1

這是可能的Postgres裏也the main difference is

Note that the target table must not appear in the from_list , unless you intend a self-join (in which case it must appear with an alias in the from_list).

您的查詢轉換：

UPDATE "Before" "T1" 
SET "T1"."Amount" = ("T1"."Amount" - "T2"."Disc") + (("DT1"."N_Amount")/2) 
FROM "Before" "T2" 
LEFT JOIN "DiscTable" "DT1" ON "T1"."PurchID" = "DT1"."Purch_ID" 
WHERE "T1"."ID" = "T2"."ID" 

但爲什麼在這裏使用自加入？ （如果"ID"是主鍵），我想你可以achive你的目標可以簡單地使用：

UPDATE "Before" "T1" 
SET "T1"."Amount" = ("T1"."Amount" - "T1"."Disc") + (("DT1"."N_Amount")/2) 
FROM "DiscTable" "DT1" 
WHERE "T1"."PurchID" = "DT1"."Purch_ID" 

編輯：about quoting：

Quoting an identifier also makes it case-sensitive, whereas unquoted names are always folded to lower case. For example, the identifiers FOO, foo, and "foo" are considered the same by PostgreSQL, but "Foo" and "FOO" are different from these three and each other. (The folding of unquoted names to lower case in PostgreSQL is incompatible with the SQL standard, which says that unquoted names should be folded to upper case. Thus, foo should be equivalent to "FOO" not "foo" according to the standard. If you want to write portable applications you are advised to always quote a particular name or never quote it.)