合速度如何比较两个数据表
有些时候，我们可能想要比较一下两个数据表，以找到其中不同的数据。比如，在进行数据移植的时候，或是在合并数据的时候，或是在比对验证数据的时候。当然比较两个表，需要这两个表结构是一样的。
我们先假设一下有如下表结构：
CREATE TABLE jajal
(
user_id integer NOT NULL,
first_name character varying(255),
last_name character varying(255),
grade character(1),
CONSTRAINT jajal_pkey PRIMARY KEY (user_id)
)
然后，我们有两张表——jajal和jajal_copy，其内容如下：
目录
jajaljajal_copy使用FULL OUTER JOIN使用NATURAL FULL OUTER JOINMySQL SQL 代码PostgreSQL 下的SQL语句
jajal
user_id
first_name
last_name
grade
1
Some
Dude
A
2
Other
Guy
B
3
You are
Welcome
B
4
What
Other
A
5
INeed
You
C
6
Mixed
Nuts
Z
7
Kirk
Land
B
8
Bit
Shooter
A
9
Sun
Microsystem
C
10
Extra
Fancy
B
jajal_copy
user_id
first_name
last_name
grade
1
Some
Dude
A
2
Other
Guy
B
3
You are
Welcome
B
4
What
Other
A
5
INeed
You
C
6
Mixed
Nuts
C
7
Kirk
Land
B
8
Bit
Shooter
A
9
Sun
Microsystem
C
10
Extra
Fancy
B
要比较这两张表的数据，找出不一样的数据行。我们可以使用outer join 技术。我给outer join做了一个链接，是Wikipedia的，如果你对这个技术不是很清楚，还请你行看看其技术细节。
下面是具体的SQL语句：
使用FULL OUTER JOIN
SELECT
*
FROM
jajal j
FULL OUTER JOIN jajal_copy jc ON jc.first_name = j.first_name
AND jc.last_name = j.last_name
AND jc.grade = j.grade
AND jc.user_id = j.user_id
WHERE
j.user_id IS NULL
OR jc.user_id IS NULL
运行结果如下：
user_id
first_name
last_name
grade
user_id
first_name
last_name
grade
[NULL]
[NULL]
[NULL]
[NULL]
6
Mixed
Nuts
C
6
Mixed
Nuts
Z
[NULL]
[NULL]
[NULL]
[NULL]
使用NATURAL FULL OUTER JOIN
关于natural join，你可以看看Wikipedia是怎么说的。
SELECT
*
FROM
jajal j
NATURAL FULL OUTER JOIN jajal_copy jc
WHERE
j.user_id IS NULL
OR jc.user_id IS NULL
运行结果如下：
user_id
first_name
last_name
grade
6
Mixed
Nuts
C
6
Mixed
Nuts
Z
MySQL SQL 代码
MySQL 并不支持 FULL OUTER JOIN，但是我们可以使用LEFT JOIN 和 RIGHT JOIN 来实现这一功能。如下所示。
SELECT
*
FROM
jajal j
LEFT JOIN jajal_copy jc ON jc.first_name = j.first_name
AND jc.last_name = j.last_name
AND jc.grade = j.grade
AND jc.user_id = j.user_id
WHERE
jc.user_id IS NULL
UNION ALL
SELECT
*
FROM
jajal j
RIGHT JOIN jajal_copy jc ON jc.first_name = j.first_name
AND jc.last_name = j.last_name
AND jc.grade = j.grade
AND jc.user_id = j.user_id
WHERE
j.user_id IS NULL
或者你更喜欢NATURAL JOIN 版本
SELECT
*
FROM
jajal j
NATURAL LEFT JOIN jajal_copy jc
WHERE
jc.user_id IS NULL
UNION ALL
SELECT
*
FROM
jajal j
NATURAL RIGHT JOIN jajal_copy jc
WHERE
j.user_id IS NULL
当然，如果你需要一个MySQL的存储过程的话，下面是一个示例：
DELIMITER $$
CREATE PROCEDURE db_schema.tablediff
(schema_name VARCHAR(64), table1 VARCHAR(64), table2 VARCHAR(64))
BEGIN
DECLARE done INT DEFAULT 0;
DECLARE sql_statement TEXT DEFAULT '';
DECLARE sql_statement_where TEXT DEFAULT '';
DECLARE sql_statement_pk TEXT DEFAULT '';
DECLARE col_name VARCHAR(64);
DECLARE col_name_cur CURSOR FOR
SELECT
COLUMN_NAME
FROM
information_schema.COLUMNS
WHERE
TABLE_SCHEMA = schema_name
AND TABLE_NAME = table1
;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = 1;
OPEN col_name_cur;
traverse_columns: LOOP
FETCH col_name_cur INTO col_name;
IF done THEN
CLOSE col_name_cur;
LEAVE traverse_columns;
END IF;
SET sql_statement_where = CONCAT(sql_statement_where,
' AND a.', col_name, ' = b.', col_name);
SET sql_statement_pk = CONCAT(sql_statement_pk,
'AND b.', col_name, ' IS NULL');
END LOOP;
SELECT
COLUMN_NAME INTO col_name
FROM
information_schema.KEY_COLUMN_USAGE
WHERE
CONSTRAINT_SCHEMA = schema_name
AND CONSTRAINT_NAME = 'PRIMARY'
AND TABLE_NAME = table1
LIMIT 1
;
IF col_name IS NOT NULL THEN
SET sql_statement_pk = CONCAT('AND b.', col_name, ' IS NULL');
END IF;
SET sql_statement = CONCAT('SELECT * FROM ', schema_name, '.', table1, ' a LEFT JOIN ', schema_name, '.', table2, ' b ON TRUE');
SET sql_statement = CONCAT(sql_statement, sql_statement_where, ' WHERE TRUE ', sql_statement_pk);
SET sql_statement = CONCAT(sql_statement, ' UNION ALL SELECT * FROM ', schema_name, '.', table1, ' b RIGHT JOIN ', schema_name, '.', table2, ' a ON TRUE');
SET sql_statement = CONCAT(sql_statement, sql_statement_where, ' WHERE TRUE ', sql_statement_pk);
SET @s = sql_statement;
PREPARE stmt1 FROM @s;
EXECUTE stmt1;
DEALLOCATE PREPARE stmt1;
END$$
DELIMITER ;
PostgreSQL 下的SQL语句
下面是PostgreSQL的一个存储过程：
CREATE OR REPLACE FUNCTION tablediff (
IN schema_name VARCHAR(64),
IN table1 VARCHAR(64),
IN table2 VARCHAR(64)
) RETURNS BIGINT AS
$BODY$
DECLARE
the_result BIGINT DEFAULT 0;
sql_statement TEXT DEFAULT '';
sql_statement_where TEXT DEFAULT '';
sql_statement_pk TEXT DEFAULT '';
col_name VARCHAR(64);
col_name_cur CURSOR FOR
SELECT
column_name
FROM
information_schema.columns
WHERE
table_catalog = schema_name
AND table_schema = 'public'
AND table_name = table1
;
BEGIN
OPEN col_name_cur;
LOOP
FETCH col_name_cur INTO col_name;
IF NOT FOUND THEN
EXIT;
END IF;
sql_statement_where := sql_statement_where || ' AND a.' || col_name || ' = b.' || col_name;
END LOOP;
SELECT
column_name INTO col_name
FROM
information_schema.table_constraints tc
JOIN information_schema.constraint_column_usage ccu ON
ccu.constraint_name = tc.constraint_name
WHERE
tc.table_catalog = schema_name
AND tc.table_schema = 'public'
AND tc.table_name = table1
LIMIT 1
;
IF col_name IS NOT NULL THEN
sql_statement_pk := ' a.' || col_name || ' IS NULL';
sql_statement_pk := sql_statement_pk || ' OR b.' || col_name || ' IS NULL';
END IF;
sql_statement := 'SELECT COUNT(*) FROM ' || schema_name || '.public.' || table1 || ' a FULL OUTER JOIN ' || schema_name || '.public.' || table2 || ' b ON TRUE';
sql_statement := sql_statement || sql_statement_where || ' WHERE ' || sql_statement_pk;
EXECUTE sql_statement INTO the_result;
RETURN the_result;
END;$BODY$
LANGUAGE 'plpgsql' STABLE;
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