SQL

由于SQL语法灵活复杂，分布式数据库和单机数据库的查询场景又不完全相同，难免有和单机数据库不兼容的SQL出现。

本文详细罗列出已明确可支持的SQL种类以及已明确不支持的SQL种类，尽量让使用者避免踩坑。

其中必然有未涉及到的SQL欢迎补充，未支持的SQL也尽量会在未来的版本中支持。

支持项

路由至单数据节点

100%全兼容（目前仅MySQL，其他数据库完善中）。

路由至多数据节点以及非MySQL

全面支持DQL、DML、DDL、DCL、TCL和MySQL的部分DAL。支持分页、去重、排序、分组、聚合、关联查询（不支持跨库关联）。以下用最为复杂的DQL举例：

SELECT主语句

SELECT select_expr [, select_expr ...] FROM table_reference [, table_reference ...]
[WHERE where_condition] 
[GROUP BY {col_name | position} [ASC | DESC]] 
[ORDER BY {col_name | position} [ASC | DESC], ...] 
[LIMIT {[offset,] row_count | row_count OFFSET offset}]

select_expr

* | 
[DISTINCT] COLUMN_NAME [AS] [alias] | 
(MAX | MIN | SUM | AVG)(COLUMN_NAME | alias) [AS] [alias] | 
COUNT(* | COLUMN_NAME | alias) [AS] [alias]

table_reference

tbl_name [AS] alias] [index_hint_list] | 
table_reference ([INNER] | {LEFT|RIGHT} [OUTER]) JOIN table_factor [JOIN ON conditional_expr | USING (column_list)] |

不支持项

路由至多数据节点以及非MySQL

不支持冗余括号、CASE WHEN、HAVING、UNION (ALL)，有限支持子查询。

除了分页子查询的支持之外(详情请参考分页)，也支持同等模式的子查询。无论嵌套多少层，ShardingSphere都可以解析至第一个包含数据表的子查询，一旦在下层嵌套中再次找到包含数据表的子查询将直接抛出解析异常。

例如，以下子查询可以支持：

SELECT COUNT(*) FROM (SELECT * FROM t_order o)

以下子查询不支持：

SELECT COUNT(*) FROM (SELECT * FROM t_order o WHERE o.id IN (SELECT id FROM t_order WHERE status = ?))

简单来说，通过子查询进行非功能需求，在大部分情况下是可以支持的。比如分页、统计总数等；而通过子查询实现业务查询当前并不能支持。

由于归并的限制，子查询中包含聚合函数目前无法支持。

不支持包含schema的SQL。因为ShardingSphere的理念是像使用一个数据源一样使用多数据源，因此对SQL的访问都是在同一个逻辑schema之上。

示例

支持的SQL

SQL	必要条件
SELECT FROM tbl_name
SELECT FROM tbl_name WHERE (col1 = ? or col2 = ?) and col3 = ?
SELECT FROM tbl_name WHERE col1 = ? ORDER BY col2 DESC LIMIT ?
SELECT COUNT(), SUM(col1), MIN(col1), MAX(col1), AVG(col1) FROM tbl_name WHERE col1 = ?
SELECT COUNT(col1) FROM tbl_name WHERE col2 = ? GROUP BY col1 ORDER BY col3 DESC LIMIT ?, ?
INSERT INTO tbl_name (col1, col2,…) VALUES (?, ?, ….)
INSERT INTO tbl_name VALUES (?, ?,….)
INSERT INTO tbl_name (col1, col2, …) VALUES (?, ?, ….), (?, ?, ….)
UPDATE tbl_name SET col1 = ? WHERE col2 = ?
DELETE FROM tbl_name WHERE col1 = ?
CREATE TABLE tbl_name (col1 int, …)
ALTER TABLE tbl_name ADD col1 varchar(10)
DROP TABLE tbl_name
TRUNCATE TABLE tbl_name
CREATE INDEX idx_name ON tbl_name
DROP INDEX idx_name ON tbl_name
DROP INDEX idx_name	TableRule中配置logic-index
SELECT DISTINCT * FROM tbl_name WHERE col1 = ?
SELECT COUNT(DISTINCT col1) FROM tbl_name

不支持的SQL

SQL	不支持原因
INSERT INTO tbl_name (col1, col2, …) SELECT col1, col2, … FROM tbl_name WHERE col3 = ?	INSERT .. SEL
INSERT INTO tbl_name SET col1 = ?	INSERT .. SET
SELECT COUNT(col1) as count_alias FROM tbl_name GROUP BY col1 HAVING count_alias > ?	HAVING
SELECT FROM tbl_name1 UNION SELECT FROM tbl_name2	UNION
SELECT FROM tbl_name1 UNION ALL SELECT FROM tbl_name2	UNION ALL
SELECT FROM tbl_name1 WHERE (val1=?) AND (val1=?)	冗余括号(MySQL数据库已支持)
SELECT FROM ds.tbl_name1	包含schema
SELECT SUM(DISTINCT col1), SUM(col1) FROM tbl_name	详见`DISTINCT`支持情况详细说明

DISTINCT支持情况详细说明

支持的SQL

SQL	所需条件
SELECT DISTINCT * FROM tbl_name WHERE col1 = ?
SELECT DISTINCT col1 FROM tbl_name
SELECT DISTINCT col1, col2, col3 FROM tbl_name
SELECT DISTINCT col1 FROM tbl_name ORDER BY col1
SELECT DISTINCT col1 FROM tbl_name ORDER BY col2
SELECT DISTINCT(col1) FROM tbl_name	MySQL
SELECT AVG(DISTINCT col1) FROM tbl_name	MySQL
SELECT SUM(DISTINCT col1) FROM tbl_name	MySQL
SELECT COUNT(DISTINCT col1) FROM tbl_name	MySQL
SELECT COUNT(DISTINCT col1) FROM tbl_name GROUP BY col1	MySQL
SELECT COUNT(DISTINCT col1 + col2) FROM tbl_name	MySQL
SELECT COUNT(DISTINCT col1), SUM(DISTINCT col1) FROM tbl_name	MySQL
SELECT COUNT(DISTINCT col1), col1 FROM tbl_name GROUP BY col1	MySQL
SELECT col1, COUNT(DISTINCT col1) FROM tbl_name GROUP BY col1	MySQL

不支持的SQL

SQL	不支持原因
SELECT SUM(DISTINCT col1), SUM(col1) FROM tbl_name	同时使用普通聚合函数和DISTINCT聚合函数