Detailed guide to compare two tables using natural full join in SQL

在SQL中,There are several ways to compare two similar tables.假设PostgreSQL的语法,We might have such a pattern：

CREATE TABLE t1 (a INT, b INT, c INT);
CREATE TABLE t2 (a INT, b INT, c INT);
INSERT INTO t1 VALUES (1, 2, 3), (4, 5, 6), (7, 8, 9);
INSERT INTO t2 VALUES            (4, 5, 6), (7, 8, 9), (10, 11, 12);
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现在可以按照Chris Saxon的建议,使用UNION 和EXCEPT ：

You can compare two tables in the following way

t1减去t2联合t2减去t1

有一个更好的方法,You query each table once......But you need to list all the columns

幸运的是,@StewAshtonA neat package has been built,为你生成#SQL:)https://t.co/7OroPV6JdY

- Chris Saxon (@ChrisRSaxon)2020年8月5日

在PostgreSQL中,我们可以这样写：

(TABLE t1 EXCEPT TABLE t2) 
UNION
(TABLE t2 EXCEPT TABLE t1)
ORDER BY a, b, c
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注意,TABLE x 只是标准的SQL,而PostgreSQL则是SELECT * FROM x 的语法糖.我们会得到：

a |b |c |
--|--|--|
 1| 2| 3|
10|11|12|

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不幸的是,This requires two accesses to each table.Can we do it with an access？

Use natural full connection

是的！使用.使用NATURAL FULL JOIN ,This is another rare use case for this esoteric operator. 假设没有NULL值,我们可以这样写：

SELECT *
FROM (
  SELECT 't1' AS t1, t1.* FROM t1
) t1 NATURAL FULL JOIN (
  SELECT 't2' AS t2, t2.* FROM t2
) t2 
WHERE NOT (t1, t2) IS NOT NULL;
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这将产生：

a |b |c |t1|t2|
--|--|--|--|--|
 1| 2| 3|t1|  |
10|11|12|  |t2|
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为什么？因为NATURAL JOIN is syntactic sugar for a join using all the shared column names of the two tables,而FULL JOIN Make sure we can also retrieve columns that are not matched by the join predicate.另一种写法是：

-- Use JOIN .. USING, instead of NATURAL JOIN
SELECT *
FROM (
  SELECT 't1' AS t1, t1.* FROM t1
) t1 FULL JOIN (
  SELECT 't2' AS t2, t2.* FROM t2
) t2 USING (a, b, c)
WHERE NOT (t1, t2) IS NOT NULL;
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或者：

-- Use JOIN .. ON, instead of JOIN .. USING
SELECT 
  coalesce(t1.a, t2.a) AS a,
  coalesce(t1.b, t2.b) AS b,
  coalesce(t1.c, t2.c) AS c,
  t1.t1,
  t2.t2
FROM (
  SELECT 't1' AS t1, t1.* FROM t1
) t1 FULL JOIN (
  SELECT 't2' AS t2, t2.* FROM t2
) t2 ON (t1.a, t1.b, t1.c) = (t2.a, t2.b, t2.c) 
WHERE NOT (t1, t2) IS NOT NULL;
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不幸的是,从PostgreSQL 12开始,This produces an error：

ERROR: FULL JOINOnly join conditions for mergeable joins or hashable joins are supported

优点和缺点

与使用UNION 和EXCEPT compared to the set operator solution,优点和缺点：优点

• Each table is accessed only once
• Comparisons are now based on names,rather than column index based,也就是说,If only some of the columns are shared,它仍然可以工作.

缺点

• If index-based column comparison is required（因为表在结构上是相同的,But don't share the exact same column names）,Then we have to rename each individual column to a common column name.
• 如果有重复的数据,There will be a Cartesian product,This can make this solution rather slow
• UNION 和 ,将 的值视为 "非独立". 则不是这种情况.See workaround belowEXCEPTNULLNATURAL JOIN

当数据中存在NULL 值时

在有NULL 值的情况下,我们不能再使用NATURAL JOIN 或JOIN .. USING .我们可以使用 [DISTINCT predicate](http://blog.jooq.org/2012/09/21/the-is-distinct-from-predicate/):

SELECT 
  coalesce(t1.a, t2.a) AS a,
  coalesce(t1.b, t2.b) AS b,
  coalesce(t1.c, t2.c) AS c,
  t1.t1,
  t2.t2
FROM (
  SELECT 't1' AS t1, t1.* FROM t1
) t1 FULL JOIN (
  SELECT 't2' AS t2, t2.* FROM t2
) t2 ON (t1.a, t1.b, t1.c) IS NOT DISTINCT FROM (t2.a, t2.b, t2.c) 
WHERE NOT (t1, t2) IS NOT NULL;
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row value expressionNULL谓词

观察The esoteric expression of row valuesNULL 谓词的用法,It uses the truth table below：

+-----------------------+-----------+---------------+---------------+-------------------+
| Expression            | R IS NULL | R IS NOT NULL | NOT R IS NULL | NOT R IS NOT NULL |
+-----------------------+-----------+---------------+---------------+-------------------+
| degree 1: null        | true      | false         | false         |  true             |
| degree 1: not null    | false     | true          | true          |  false            |
| degree > 1: all null  | true      | false         | false         |  true             |
| degree > 1: some null | false     | false         | true          |  true             |
| degree > 1: none null | false     | true          | true          |  false            |
+-----------------------+-----------+---------------+---------------+-------------------+
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是的.R IS NULL 和NOT R IS NOT NULL 在SQLis not the same thing...这只是另一种写法：

SELECT *
FROM (
  SELECT 't1' AS t1, t1.* FROM t1
) t1 NATURAL FULL JOIN (
  SELECT 't2' AS t2, t2.* FROM t2
) t2 
WHERE t1 IS NULL
OR t2 IS NULL;
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