子查询

本文介绍PolarDB-X支持的子查询类别及在PolarDB-X中使用子查询的相关限制和注意事项。

使用限制

相比原生MySQL，PolarDB-X在子查询使用上增加了如下限制：

不支持在HAVING子句中使用子查询，示例如下：

SELECT name, AVG( quantity )
FROM tb1
GROUP BY name
HAVING AVG( quantity ) > 2* (
   SELECT AVG( quantity )
   FROM tb2
 );

不支持在JOIN ON子句中使用子查询，示例如下：

SELECT * FROM tb1 p JOIN tb2 s on (p.id=s.id and p.quantity>All(select quantity from tb3))

等号操作行符的标量子查询（The Subquery as Scalar Operand）不支持ROW语法。示例如下：
```
select * from tb1 where row(id, name) = (select id, name from tb2)
```

不支持在UPDATE SET子句中使用子查询，示例如下：

UPDATE t1 SET c1 = (SELECT c2 FROM t2 WHERE t1.c1 = t2.c1) LIMIT 10

注意事项

PolarDB-X中部分子查询仅能以APPLY的方式执行，查询效率低下。在实际使用中请尽量避免如下例子中的低效SQL：

WHERE条件中OR与子查询共存时，执行效率会依外表数据情况大幅降低。示例如下：

高效：select * from tb1 where id in (select id from tb2)
高效：select * from tb1 where id in (select id from tb2) and id>3
低效：select * from tb1 where id in (select id from tb2) or  id>3

关联子查询（Correlated Subqueries）的关联项中带函数或非等号运算符。示例如下：

高效：select * from tb1 a where id in
      (select id from tb2 b where a.name=b.name)
低效：select * from tb1 a where id in
      (select id from tb2 b where UPPER(a.name)=b.name)
低效：select * from tb1 a where id in
      (select id from tb2 b where a.decimal_test=abs(b.decimal_test))
低效：select * from tb1 a where id in
      (select id from tb2 b where a.name!=b.name)
低效：select * from tb1 a where id in
      (select id from tb2 b where a.name>=b.name)

关联子查询（Correlated Subqueries）关联项与其它条件的逻辑运算符为OR。示例如下：

高效：select * from tb1 a where id in
      (select id from tb2 b where a.name=b.name
                                  and b.date_test<'2015-12-02')
低效：select * from tb1 a where id in
      (select id from tb2 b where a.name=b.name
                                  or b.date_test<'2015-12-02')
低效：select * from tb1 a where id in
      (select id from tb2 b where a.name=b.name
                                  or b.date_test=a.date_test)

标量子查询（The Subquery as Scalar Operand）带关联项。示例如下：

高效：select * from tb1 a where id >
        (select id from tb2 b where b.date_test<'2015-12-02')
低效：select * from tb1 a where id >
        (select id from tb2 b where a.name=b.name 
                                    and b.date_test<'2015-12-02')

跨关联层子查询。示例如下：

SQL多层关联，每层子查询关联项仅与直接上层关联，此类高效。

高效：select * from tb1 a where id in(select id from tb2 b 
where a.name=b.name and 
exists (select name from tb3 c where b.address=c.address))

SQL多层关联，但表c的子查询关联项中与表a的列进行了关联，此类低效。

低效：select * from tb1 a where id in(select id from tb2 b 
where a.name=b.name and 
exists (select name from tb3 c where a.address=c.address))

说明上述示例中，表a和表b、表b和表c为直接层级关联，表a和表c间为跨层关联。

子查询中包含GROUP BY，请确保GROUP BY的分组列包含关联项。示例如下：

SQL子查询中包含聚合函数和关联项，关联项b.pk包含于分组列pk之中，此类高效。

高效：select * from tb1 a where exists 
    (select pk from tb2 b 
                where a.pk=b.pk and  b.date_test='2003-04-05' 
                group by pk);

SQL子查询中包含聚合函数和关联项，关联项b.date_test不包含于分组列pk之中，此类低效。

低效：select * from tb1 a where exists 
    (select pk from tb2 b 
                where a.date_test=b.date_test and b.date_test='2003-04-05' 
                group by pk);

支持的子查询

PolarDB-X目前支持如下类别的子查询：

Comparisons Using SubqueriesComparisons Using Subqueries指带有比较运算符的子查询，这类子查询最为常见。
- 语法
```
non_subquery_operand comparison_operator (subquery)
comparison_operator： =  >  <  >=  <=  <>  !=  <=> like
```

示例
```
select * from tb1 WHERE 'a' = (SELECT column1 FROM t1)
```
说明目前仅支持子查询在比较运算符的右边。

Subqueries with ANY、ALL、IN/NOT IN、EXISTS/NOT EXISTS

语法

operand comparison_operator ANY (subquery)
operand comparison_operator ALL (subquery)
operand IN (subquery)
operand NOT IN (subquery)
operand EXISTS (subquery)
operand NOT EXISTS (subquery)
comparison_operator:=  >  <  >=  <=  <>  !=

示例
- ANY：如果子查询返回的任意一行满足ANY前的表达式，返回TRUE，否则返回FALSE。
- ALL：如果子查询返回所有行都满足ALL前的表达式，返回TRUE，否则返回FALSE。
- IN：在子查询前使用时，IN等价于=ANY。示例如下：
```
SELECT s1 FROM t1 WHERE s1 = ANY (SELECT s1 FROM t2);
SELECT s1 FROM t1 WHERE s1 IN    (SELECT s1 FROM t2);
```
- NOT IN：NOT IN在子查询前使用时，等价于<>ALL。示例如下：
```
SELECT s1 FROM t1 WHERE s1 <> ALL (SELECT s1 FROM t2);
SELECT s1 FROM t1 WHERE s1 NOT IN (SELECT s1 FROM t2);
```
- EXISTS：如果子查询返回任意行，EXISTS子查询结果为TRUE；如果子查询返回空值，EXISTS子查询结果为FALSE。示例如下：
```
SELECT column1 FROM t1 WHERE EXISTS (SELECT * FROM t2);
```
  说明如果EXISTS子查询中包含任意行，即使只包含NULL的行值，WHERE条件也会返回TRUE。
- NOT EXISTS：如果子查询返回任意行，NOT EXISTS子查询结果为FALSE；如果子查询返回空值，NOT EXISTS子查询结果为TRUE。

Row Subqueries
- Row Subqueries支持如下比较运算符：
```
comparison_operator：=  >  <  >=  <=  <>  !=  <=>
```

示例
```
SELECT * FROM t1
  WHERE (col1,col2) = (SELECT col3, col4 FROM t2 WHERE id = 10);
SELECT * FROM t1
  WHERE ROW(col1,col2) = (SELECT col3, col4 FROM t2 WHERE id = 10);
```
以上两个SQL是等价的，只有同时满足以下条件时，t1表的数据行才会返回：
- 子查询（SELECT col3, col4 FROM t2 WHERE id=10）仅返回一行记录，返回多行会报错。
- 子查询返回的col3，col4结果与主表中col1，col2的值需一一对应。

Correlated SubqueriesCorrelated Subqueries指子查询中包含对外层查询表的引用。示例如下：

SELECT * FROM t1
  WHERE column1 = ANY (SELECT column1 FROM t2
                       WHERE t2.column2 = t1.column2);

示例子查询SQL中并没有包含表t1及其列名column2，此时会向上一层寻找表t1的引用。

Derived Tables（Subqueries in the FROM Clause）Derived Tables指在FROM子句中的子查询。
- 语法
```
SELECT ... FROM (subquery) [AS] tbl_name ...
```

示例
1. 数据准备：使用如下语法创建表t1：
```
CREATE TABLE t1 (s1 INT, s2 CHAR(5), s3 FLOAT);
INSERT INTO t1 VALUES (1,'1',1.0);
INSERT INTO t1 VALUES (2,'2',2.0);
```
  使用如下查询并得到查询结果为2, '2', 4.0。
```
SELECT sb1,sb2,sb3
FROM (SELECT s1 AS sb1, s2 AS sb2, s3*2 AS sb3 FROM t1) AS sb
WHERE sb1 > 1;
```
2. 查询需求：获取分组数据SUM后的平均值。若直接使用如下SQL则会报错，无法执行：
```
SELECT AVG(SUM(s1)) FROM t1 GROUP BY s1;
```
  此时可使用如下Derived Tables子查询，并得到查询结果为1.5000：
```
SELECT AVG(sum_s1)
  FROM (SELECT SUM(s1) AS sum_s1
        FROM t1 GROUP BY s1) AS t1;
```
  说明
  - Derived Tables必须拥有一个别名（如示例中的t1）。
  - Derived Tables可以返回一个标量、列、行或表。
  - Derived Tables不可以成为Correlated Subqueries，即不能包含子查询外部表的引用。

子查询

子查询

使用限制

注意事项

支持的子查询

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