子查询
本篇文档向你介绍 MatrixOne 的子查询功能。
概述
子查询是嵌套在另一个查询中的 SQL 表达式,借助子查询,可以在一个查询当中使用另外一个查询的查询结果。
通常情况下,从 SQL 语句结构上,子查询语句一般有以下几种形式:
- 标量子查询(Scalar Subquery),如
SELECT (SELECT s1 FROM t2) FROM t1
。 - 派生表(Derived Tables),如
SELECT t1.s1 FROM (SELECT s1 FROM t2) t1
。 - 存在性测试(Existential Test),如
WHERE NOT EXISTS(SELECT ... FROM t2)
,WHERE t1.a IN (SELECT ... FROM t2)
。 - 集合比较(Quantified Comparison),如
WHERE t1.a = ANY(SELECT ... FROM t2)
。 - 作为比较运算符操作数的子查询,如
WHERE t1.a > (SELECT ... FROM t2)
。
关于子查询 SQL 语句,参见 SUBQUERY。
另外,从 SQL 语句执行情况上,子查询语句一般有以下两种形式:
-
关联子查询(Correlated Subquery):数据库嵌套查询中内层查询和外层查询不相互独立,内层查询也依赖于外层查询。执行顺序为:
-
先从外层查询中查询中一条记录。
-
再将查询到的记录放到内层查询中符合条件的记录,再放到外层中查询。
-
重复以上步骤
例如:
select * from tableA where tableA.cloumn < (select column from tableB where tableA.id = tableB.id))
-
-
无关联子查询 (Self-contained Subquery):数据库嵌套查询中内层查询是完全独立于外层查询的。执行顺序为:
- 先执行内层查询。
- 得到内层查询的结果后带入外层,再执行外层查询。
例如:
select * from tableA where tableA.column = (select tableB.column from tableB )
子查询的作用:
- 子查询允许结构化的查询,这样就可以把一个查询语句的每个部分隔开。
- 子查询提供了另一种方法来执行有些需要复杂的
JOIN
和UNION
来实现的操作。
我们将举一个简单的例子帮助你理解关联子查询和无关联子查询。
示例
开始前准备
你需要确认在开始之前,已经完成了以下任务:
- 已完成单机部署 MatrixOne。
数据准备
-
下载数据集:
https://community-shared-data-1308875761.cos.ap-beijing.myqcloud.com/tpch/tpch-1g.zip
-
创建数据库和数据表:
create database d1; use d1; CREATE TABLE NATION ( N_NATIONKEY INTEGER NOT NULL, N_NAME CHAR(25) NOT NULL, N_REGIONKEY INTEGER NOT NULL, N_COMMENT VARCHAR(152), PRIMARY KEY (N_NATIONKEY)); CREATE TABLE REGION ( R_REGIONKEY INTEGER NOT NULL, R_NAME CHAR(25) NOT NULL, R_COMMENT VARCHAR(152), PRIMARY KEY (R_REGIONKEY)); CREATE TABLE PART ( P_PARTKEY INTEGER NOT NULL, P_NAME VARCHAR(55) NOT NULL, P_MFGR CHAR(25) NOT NULL, P_BRAND CHAR(10) NOT NULL, P_TYPE VARCHAR(25) NOT NULL, P_SIZE INTEGER NOT NULL, P_CONTAINER CHAR(10) NOT NULL, P_RETAILPRICE DECIMAL(15,2) NOT NULL, P_COMMENT VARCHAR(23) NOT NULL, PRIMARY KEY (P_PARTKEY)); CREATE TABLE SUPPLIER ( S_SUPPKEY INTEGER NOT NULL, S_NAME CHAR(25) NOT NULL, S_ADDRESS VARCHAR(40) NOT NULL, S_NATIONKEY INTEGER NOT NULL, S_PHONE CHAR(15) NOT NULL, S_ACCTBAL DECIMAL(15,2) NOT NULL, S_COMMENT VARCHAR(101) NOT NULL, PRIMARY KEY (S_SUPPKEY)); CREATE TABLE PARTSUPP ( PS_PARTKEY INTEGER NOT NULL, PS_SUPPKEY INTEGER NOT NULL, PS_AVAILQTY INTEGER NOT NULL, PS_SUPPLYCOST DECIMAL(15,2) NOT NULL, PS_COMMENT VARCHAR(199) NOT NULL, PRIMARY KEY (PS_PARTKEY, PS_SUPPKEY)); CREATE TABLE CUSTOMER ( C_CUSTKEY INTEGER NOT NULL, C_NAME VARCHAR(25) NOT NULL, C_ADDRESS VARCHAR(40) NOT NULL, C_NATIONKEY INTEGER NOT NULL, C_PHONE CHAR(15) NOT NULL, C_ACCTBAL DECIMAL(15,2) NOT NULL, C_MKTSEGMENT CHAR(10) NOT NULL, C_COMMENT VARCHAR(117) NOT NULL, PRIMARY KEY (C_CUSTKEY)); CREATE TABLE ORDERS ( O_ORDERKEY BIGINT NOT NULL, O_CUSTKEY INTEGER NOT NULL, O_ORDERSTATUS CHAR(1) NOT NULL, O_TOTALPRICE DECIMAL(15,2) NOT NULL, O_ORDERDATE DATE NOT NULL, O_ORDERPRIORITY CHAR(15) NOT NULL, O_CLERK CHAR(15) NOT NULL, O_SHIPPRIORITY INTEGER NOT NULL, O_COMMENT VARCHAR(79) NOT NULL, PRIMARY KEY (O_ORDERKEY)); CREATE TABLE LINEITEM ( L_ORDERKEY BIGINT NOT NULL, L_PARTKEY INTEGER NOT NULL, L_SUPPKEY INTEGER NOT NULL, L_LINENUMBER INTEGER NOT NULL, L_QUANTITY DECIMAL(15,2) NOT NULL, L_EXTENDEDPRICE DECIMAL(15,2) NOT NULL, L_DISCOUNT DECIMAL(15,2) NOT NULL, L_TAX DECIMAL(15,2) NOT NULL, L_RETURNFLAG CHAR(1) NOT NULL, L_LINESTATUS CHAR(1) NOT NULL, L_SHIPDATE DATE NOT NULL, L_COMMITDATE DATE NOT NULL, L_RECEIPTDATE DATE NOT NULL, L_SHIPINSTRUCT CHAR(25) NOT NULL, L_SHIPMODE CHAR(10) NOT NULL, L_COMMENT VARCHAR(44) NOT NULL, PRIMARY KEY (L_ORDERKEY, L_LINENUMBER));
-
把数据导入到数据表中:
load data infile '/YOUR_TPCH_DATA_PATH/nation.tbl' into table NATION FIELDS TERMINATED BY '|' OPTIONALLY ENCLOSED BY '"' LINES TERMINATED BY '\n'; load data infile '/YOUR_TPCH_DATA_PATH/region.tbl' into table REGION FIELDS TERMINATED BY '|' OPTIONALLY ENCLOSED BY '"' LINES TERMINATED BY '\n'; load data infile '/YOUR_TPCH_DATA_PATH/part.tbl' into table PART FIELDS TERMINATED BY '|' OPTIONALLY ENCLOSED BY '"' LINES TERMINATED BY '\n'; load data infile '/YOUR_TPCH_DATA_PATH/supplier.tbl' into table SUPPLIER FIELDS TERMINATED BY '|' OPTIONALLY ENCLOSED BY '"' LINES TERMINATED BY '\n'; load data infile '/YOUR_TPCH_DATA_PATH/partsupp.tbl' into table PARTSUPP FIELDS TERMINATED BY '|' OPTIONALLY ENCLOSED BY '"' LINES TERMINATED BY '\n'; load data infile '/YOUR_TPCH_DATA_PATH/orders.tbl' into table ORDERS FIELDS TERMINATED BY '|' OPTIONALLY ENCLOSED BY '"' LINES TERMINATED BY '\n'; load data infile '/YOUR_TPCH_DATA_PATH/customer.tbl' into table CUSTOMER FIELDS TERMINATED BY '|' OPTIONALLY ENCLOSED BY '"' LINES TERMINATED BY '\n'; load data infile '/YOUR_TPCH_DATA_PATH/lineitem.tbl' into table LINEITEM FIELDS TERMINATED BY '|' OPTIONALLY ENCLOSED BY '"' LINES TERMINATED BY '\n';
现在你可以使用这些数据进行查询。
无关联子查询
对于将子查询作为比较运算符 (>
/ >=
/ <
/ <=
/ =
/ !=
) 操作数的这类无关联子查询而言,内层子查询只需要进行一次查询,MatrixOne 在生成执行计划阶段会将内层子查询改写为常量。
mysql> select p.p_name from (select * from part where p_brand='Brand#21' and p_retailprice between 1100 and 1200) p, partsupp ps where p.p_partkey=ps.ps_partkey and p.p_name like '%pink%' limit 10;
在 MatrixOne 执行上述查询的时候会先执行一次内层子查询:
mysql> select * from part where p_brand='Brand#21' and p_retailprice between 1100 and 1200
运行结果为:
+-----------------------------------+
| p_name |
+-----------------------------------+
| olive chartreuse smoke pink tan |
| olive chartreuse smoke pink tan |
| olive chartreuse smoke pink tan |
| olive chartreuse smoke pink tan |
| pink sienna dark bisque turquoise |
| pink sienna dark bisque turquoise |
| pink sienna dark bisque turquoise |
| pink sienna dark bisque turquoise |
| honeydew orchid cyan magenta pink |
| honeydew orchid cyan magenta pink |
+-----------------------------------+
10 rows in set (0.06 sec)
对于存在性测试和集合比较两种情况下的无关联列子查询,MatrixOne 会将其进行改写和等价替换以获得更好的执行性能。
关联子查询
对于关联子查询而言,由于内层的子查询引用外层查询的列,子查询需要对外层查询得到的每一行都执行一遍,也就是说假设外层查询得到一千万的结果,那么子查询也会被执行一千万次,这会导致查询需要消耗更多的时间和资源。
因此在处理过程中,MatrixOne 会尝试对关联子查询去关联,以从执行计划层面上提高查询效率。
mysql> select p_name from part where P_PARTKEY in (select PS_PARTKEY from PARTSUPP where PS_SUPPLYCOST>=500) and p_name like '%pink%' limit 10;
MatrixOne 在处理该 SQL 语句是会将其改写为等价的 JOIN
查询:
select p_name from part join partsupp on P_PARTKEY=PS_PARTKEY where PS_SUPPLYCOST>=500 and p_name like '%pink%' limit 10;
运行结果为:
+------------------------------------+
| p_name |
+------------------------------------+
| papaya red almond hot pink |
| turquoise hot smoke green pink |
| purple cornsilk red pink floral |
| pink cyan purple white burnished |
| sandy dark pink indian cream |
| powder cornsilk chiffon slate pink |
| rosy light black pink orange |
| pink white goldenrod ivory steel |
| cornsilk dim pink tan sienna |
| lavender navajo steel sandy pink |
+------------------------------------+
10 rows in set (0.23 sec)
作为最佳实践,在实际开发当中,为提高计算效率,尽量选择等价计算方法进行查询,避免使用关联子查询的方式进行查询。