SQL查詢左外連接到當前查詢

Question

我有一個查詢，我想查找其他表中的值作為參考，但不要弄亂我當前的查詢結果。 我認為我必須使用外部左聯接，但不確定如何將其與當前查詢合並。

我當前的查詢與此類似：

SELECT a.primary_key,
       a.phase,
       b.project_number,
       c.LENGTH,
       d.color
  FROM TableA a,
       TableB b,
       TableC c,
       TableD d
 WHERE     c.primary_key = a.PROJECT_ID
       AND b.primary_key = a.PROJECT_ID
       AND b.primary_key = d.project_ID
       AND (c.date IS NULL OR c.number IS NULL)
       AND d.color IN ('black','red','blue')
ORDER BY 1

現在，這給了我一張包含50個結果的表格。 “ TableContacts”具有對我的b.project_number的查找值。 所以說我的50個結果表，其中只有10個具有b.project_number，我需要來自'TableContacts'的查找值也可以顯示在我的結果中，但是我不希望那影響我的結果並將其縮減為10 ，我仍然需要原始的50個結果，以及這些附加信息。 救命？

Answer 1

只需將CONTACTS表添加到您的聯接中：

SELECT a.primary_key,
       a.phase,
       b.project_number,
       c.LENGTH,
       d.color, 
       ct.lookup_value --<< this is from the CONTACTS table
FROM TableA a
  JOIN TableB b ON b.primary_key = a.PROJECT_ID
  JOIN TableC c ON c.primary_key = a.PROJECT_ID
  JOIN TableD d ON b.primary_key = d.project_ID
  LEFT JOIN contacts ct ON ct.some_column = b.project_Number --<< this is the outer join to the CONTACTS table
WHERE (c.date IS NULL OR c.number IS NULL)
  AND d.color IN ('black','red','blue')
ORDER BY 1

當您混淆表名和列名時，很難猜測CONTACTS表上的CONTACTS條件應該是什么樣子。

Answer 2

您可以使用這種方法：

用當前查詢的結果創建一個表：

create table t1 as 
SELECT a.primary_key,
       a.phase,
       b.project_number,
       c.LENGTH_col,
       d.color
FROM a,b,c,d
WHERE c.primary_key = a.PROJECT_ID
       AND b.primary_key = a.PROJECT_ID
       AND b.primary_key = d.project_ID
       AND (c.date_col IS NULL OR c.number_col IS NULL)
       AND d.color IN ('black','red','blue')
ORDER BY 1;

使用聯合獲得所需的結果：

select t1.primary_key, t1.phase, t1.project_number, t1.length_col, t1.color, TableContacts.lookup_column
from t1, TableContacts
where t1.project_number = TableContacts.project_number
UNION
select t1.primary_key, t1.phase, t1.project_number, t1.length_col, t1.color, null
from t1 where t1.project_number is null;

通過創建偽數據進行說明：

select * from a;
PRIMARY_KEY | PROJECT_ID | PHASE
1   100 Phase-1
2   200 Phase-2
3   300 Phase-3
4   400 Phase-4
5   500 Phase-5

select * from b;
PRIMARY_KEY | PROJECT_NUMBER
100 null
200 2000
300 3000
400 null
500 5000

select * from c;
PRIMARY_KEY | NUMBER_COL | LENGTH_COL | DATE_COL
100 null    99  null
200 null    99  null
300 null    99  null
400 null    99  null
500 null    99  null

select * from d;
PROJECT_ID  | COLOR
100 black
200 red
300 blue
400 black
500 yellow

select * from TableContacts;
PROJECT_NUMBER  | LOOKUP_COLUMN
1000    l-1000
2000    l-2000
3000    l-3000
4000    l-4000
5000    l-5000

有問題的現有查詢返回以下內容：

SELECT a.primary_key,
       a.phase,
       b.project_number,
       c.LENGTH_col,
       d.color
FROM a,b,c,d
WHERE c.primary_key = a.PROJECT_ID
       AND b.primary_key = a.PROJECT_ID
       AND b.primary_key = d.project_ID
       AND (c.date_col IS NULL OR c.number_col IS NULL)
       AND d.color IN ('black','red','blue')
ORDER BY 1;

PRIMARY_KEY | PHASE | PROJECT_NUMBER | LENGTH_COL | COLOR
1   Phase-1 null    99  black
2   Phase-2 2000    99  red
3   Phase-3 3000    99  blue
4   Phase-4 null    99  black

目的是填充project_number不為null的lookup_column 。 運行答案開頭提供的聯合查詢：

select t1.primary_key, t1.phase, t1.project_number, t1.length_col, t1.color, TableContacts.lookup_column
from t1, TableContacts
where t1.project_number = TableContacts.project_number
UNION
select t1.primary_key, t1.phase, t1.project_number, t1.length_col, t1.color, null
from t1 where t1.project_number is null;

PRIMARY_KEY | PHASE | PROJECT_NUMBER | LENGTH_COL | COLOR | LOOKUP_COLUMN
1   Phase-1 null    99  black   null
2   Phase-2 2000    99  red     l-2000
3   Phase-3 3000    99  blue    l-3000
4   Phase-4 null    99  black   null