Question

在MySQL中执行子句的预定义顺序是什么？有些是在运行时决定的，这个顺序是否正确？

FROM clause
WHERE clause
GROUP BY clause
HAVING clause
SELECT clause
ORDER BY clause

Answer 1

MySQL语句的实际执行有点棘手。但是，该标准确实指定了查询中元素的解释顺序。这基本上是您指定的顺序，但我认为HAVING和GROUP BY可能会在SELECT之后出现：

FROM条款
WHERE条款
SELECT条款
GROUP BY条款
HAVING条款
ORDER BY条款

这对于理解如何解析查询非常重要。例如，您不能在SELECT子句中使用WHERE中定义的列别名，因为WHERE在SELECT之前被解析。另一方面，这样的别名可以在ORDER BY子句中。

至于实际执行，这实际上是由优化器决定的。例如：

. . .
GROUP BY a, b, c
ORDER BY NULL

和

. . .
GROUP BY a, b, c
ORDER BY a, b, c

两者都具有ORDER BY根本没有执行的效果 - 因此在GROUP BY之后没有执行（在第一种情况下，效果是从{{1}中删除排序而在第二个效果是只做GROUP BY已经做的事情。

Answer 2

这是你如何大致了解mysql如何执行选择查询

DROP TABLE if exists new_table;

CREATE TABLE `new_table` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`testdecimal` decimal(6,2) DEFAULT NULL,
PRIMARY KEY (`id`));

INSERT INTO `new_table` (`testdecimal`) VALUES ('1234.45');
INSERT INTO `new_table` (`testdecimal`) VALUES ('1234.45');

set @mysqlorder := '';

select @mysqlorder := CONCAT(@mysqlorder," SELECT ") from new_table,(select @mysqlorder := CONCAT(@mysqlorder," FROM ")) tt
JOIN (select @mysqlorder := CONCAT(@mysqlorder," JOIN1 ")) t on ((select @mysqlorder := CONCAT(@mysqlorder," ON1 ")) or rand() < 1)
JOIN (select @mysqlorder := CONCAT(@mysqlorder," JOIN2 ")) t2 on ((select @mysqlorder := CONCAT(@mysqlorder," ON2 ")) or rand() < 1)
where ((select @mysqlorder := CONCAT(@mysqlorder," WHERE ")) or IF(new_table.testdecimal = 1234.45,true,false))
group by (select @mysqlorder := CONCAT(@mysqlorder," GROUPBY ")),id
having (select @mysqlorder := CONCAT(@mysqlorder," HAVING "))
order by (select @mysqlorder := CONCAT(@mysqlorder," ORDERBY "));

select @mysqlorder;

这里是mysql查询的输出，希望你能搞清楚 SELECT 查询的mysql执行： -

FROM JOIN1 JOIN2 WHERE ON2 ON1 ORDERBY GROUPBY SELECT WHERE ON2 ON1 ORDERBY GROUPBY SELECT HAVING

Answer 3

我认为执行顺序是这样的：

(7)     SELECT 
(8)     DISTINCT <select_list>
(1)     FROM <left_table>
(3)     <join_type> JOIN <right_table>
(2)     ON <join_condition>
(4)     WHERE <where_condition>
(5)     GROUP BY <group_by_list>
(6)     HAVING <having_condition>
(9)     ORDER BY <order_by_condition>
(10)    LIMIT <limit_number>[, <offset_number>]

MySQL查询/子句执行顺序

3 个答案: