本文介绍DRDS支持的子查询类别及在DRDS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,DRDS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

DRDS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表b表b表c为直接层级关联,表a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

DRDS目前支持如下类别的子查询:

  • Comparisons Using Subqueries

    Comparisons 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col3col4结果与主表中col1col2的值需一一对应。
  • Correlated Subqueries

    Correlated 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。