抽样查询
描述
TABLESAMPLE 语句用于对表进行抽样。它支持以下抽样方法
TABLESAMPLE(xROWS): 将表抽样到给定行数。TABLESAMPLE(xPERCENT): 将表抽样到给定百分比。请注意，百分比定义为 0 到 100 之间的数字。TABLESAMPLE(BUCKETxOUT OFy): 将表抽样到y分之x的比例。
注意： TABLESAMPLE 返回请求的近似行数或比例。
语法
TABLESAMPLE ({ integer_expression | decimal_expression } PERCENT)
| TABLESAMPLE ( integer_expression ROWS )
| TABLESAMPLE ( BUCKET integer_expression OUT OF integer_expression )
示例
SELECT * FROM test;
+--+----+
|id|name|
+--+----+
| 5|Alex|
| 8|Lucy|
| 2|Mary|
| 4|Fred|
| 1|Lisa|
| 9|Eric|
|10|Adam|
| 6|Mark|
| 7|Lily|
| 3|Evan|
+--+----+
SELECT * FROM test TABLESAMPLE (50 PERCENT);
+--+----+
|id|name|
+--+----+
| 5|Alex|
| 2|Mary|
| 4|Fred|
| 9|Eric|
|10|Adam|
| 3|Evan|
+--+----+
SELECT * FROM test TABLESAMPLE (5 ROWS);
+--+----+
|id|name|
+--+----+
| 5|Alex|
| 8|Lucy|
| 2|Mary|
| 4|Fred|
| 1|Lisa|
+--+----+
SELECT * FROM test TABLESAMPLE (BUCKET 4 OUT OF 10);
+--+----+
|id|name|
+--+----+
| 8|Lucy|
| 2|Mary|
| 9|Eric|
| 6|Mark|
+--+----+