我可以编写不带 OR IS NULL 的 FULL OUTER JOIN 吗？

Question

这里有一些可以使用的数据：

CREATE TABLE a (
    a_id   int  NOT NULL,
    a_prop text NOT NULL
);

CREATE TABLE b (
    b_id   int  NOT NULL,
    b_prop text NOT NULL
);

INSERT INTO a VALUES (1, 'blah'), (2, 'blah'), (4, 'not this one');
INSERT INTO b VALUES (1, 'blah'), (3, 'blah'), (5, 'not this one');

现在我想编写一个返回的查询：

一种可能性是：

SELECT *
FROM a
FULL OUTER JOIN b ON a_id = b_id
WHERE (a_prop = 'blah' OR a_prop IS NULL)
AND (b_prop = 'blah' OR b_prop IS NULL);

OR ... IS NULL这要求我为我有条件的每个字段都写。如果某些条件是日期范围等，这会变得更加冗长。

如果这是左连接：

SELECT *
FROM a
LEFT JOIN b ON a_id = b_id
WHERE a_prop = 'blah'
AND (b_prop = 'blah' OR b_prop IS NULL);

我可以将条件移至ON子句中以避免这种情况：

SELECT *
FROM a
LEFT JOIN b ON a_id = b_id AND b_prop = 'blah'
WHERE a_prop = 'blah';

有没有办法通过完整的外连接来做到这一点？

小提琴

Answer 1

没有OR IS NULL？

col = 'x' OR col IS NULL

原来的：

SELECT *
FROM   a
FULL   JOIN b ON a_id = b_id
WHERE (a_prop = 'blah' OR a_prop IS NULL)
AND   (b_prop = 'blah' OR b_prop IS NULL);

使用col <> x IS NOT TRUE：

SELECT *
FROM   a
FULL   JOIN b ON a_id = b_id
WHERE  a_prop <> 'blah' IS NOT TRUE
AND    b_prop <> 'blah' IS NOT TRUE;

或者在加入之前过滤：

SELECT *
FROM      (SELECT * FROM a WHERE a_prop = 'blah') a
FULL JOIN (SELECT * FROM b WHERE b_prop = 'blah') b ON a_id = b_id;

col <> 'x' OR col IS NULL

问题的第一个版本要求这个谓词。

原来的：

SELECT *
FROM a
FULL OUTER JOIN b ON a_id = b_id
WHERE (a_prop <> 'not this one' OR a_prop IS NULL)
AND   (b_prop <> 'not this one' OR b_prop IS NULL);

使用col IS DISTINCT FROM 'x'：

SELECT *
FROM   a
FULL   JOIN b ON a_id = b_id
WHERE  a_prop IS DISTINCT FROM 'not this one'
AND    b_prop IS DISTINCT FROM 'not this one';

或者在加入之前过滤：

SELECT *
FROM      (SELECT * FROM a WHERE a_prop <> 'not this one') a
FULL JOIN (SELECT * FROM b WHERE b_prop <> 'not this one') b ON a_id = b_id;

db<>fiddle这里- 显示所有

!=旁白：我没有使用<>，它是 SQL 中的标准运算符。（!=是 Postgres 中可接受的别名。）