SELECT
SELECT 语句用于从 TiDB 读取数据。
语法图
SelectStmt:
FromDual:
WhereClauseOptional:
SelectStmtOpts:
SelectStmtFieldList:
TableRefsClause:
- TableRefsClause
- AsOfClause
TableRefsClause ::=
TableRef AsOfClause? ( ',' TableRef AsOfClause? )*
AsOfClause ::=
'AS' 'OF' 'TIMESTAMP' Expression
SelectStmtGroup:
HavingClause:
OrderByOptional:
SelectStmtLimit:
FirstOrNext:
FetchFirstOpt:
RowOrRows:
SelectLockOpt:
- SelectLockOpt
- TableList
SelectLockOpt ::=
( ( 'FOR' 'UPDATE' ( 'OF' TableList )? 'NOWAIT'? )
| ( 'LOCK' 'IN' 'SHARE' 'MODE' ) )?
TableList ::=
TableName ( ',' TableName )*
WindowClauseOptional
TableSampleOpt
- TableSampleOpt
TableSampleOpt ::=
'TABLESAMPLE' 'REGIONS()'
语法元素说明
示例
CREATE TABLE t1 (id INT NOT NULL PRIMARY KEY AUTO_INCREMENT, c1 INT NOT NULL);
Query OK, 0 rows affected (0.11 sec)
INSERT INTO t1 (c1) VALUES (1),(2),(3),(4),(5);
Query OK, 5 rows affected (0.03 sec)
Records: 5 Duplicates: 0 Warnings: 0
SELECT * FROM t1;
+----+----+
| id | c1 |
+----+----+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
| 4 | 4 |
| 5 | 5 |
+----+----+
5 rows in set (0.00 sec)
下面这个例子使用 tiup bench tpcc prepare 生成的数据，其中第一个查询展示了 TABLESAMPLE 的用法。
mysql> SELECT AVG(s_quantity), COUNT(s_quantity) FROM stock TABLESAMPLE REGIONS();
+-----------------+-------------------+
| AVG(s_quantity) | COUNT(s_quantity) |
+-----------------+-------------------+
| 59.5000 | 4 |
+-----------------+-------------------+
1 row in set (0.00 sec)
mysql> SELECT AVG(s_quantity), COUNT(s_quantity) FROM stock;
+-----------------+-------------------+
| AVG(s_quantity) | COUNT(s_quantity) |
+-----------------+-------------------+
| 54.9729 | 1000000 |
+-----------------+-------------------+
1 row in set (0.52 sec)
MySQL 兼容性
- 不支持
SELECT ... INTO @variable语法。 - 不支持
SELECT ... GROUP BY ... WITH ROLLUP语法。 - 不支持 MySQL 5.7 中支持的
SELECT .. GROUP BY expr语法，而是匹配 MySQL 8.0 的行为，不按照默认的顺序进行排序。 SELECT ... TABLESAMPLE ...是 TiDB 的扩展语法，MySQL 不支持该语法。