采样查询Sampling queries

TABLESAMPLE语句用于对表进行采样。The TABLESAMPLE statement is used to sample the table. 它支持以下采样方法:It supports the following sampling methods:

  • TABLESAMPLE (x ROWS) :将表采样到给定的行数。TABLESAMPLE(x ROWS): Sample the table down to the given number of rows.
    • TABLESAMPLE (x PERCENT) :将表采样到给定百分比。TABLESAMPLE(x PERCENT): Sample the table down to the given percentage. 请注意,百分比定义为介于0到100之间的数字。Note that percentages are defined as a number between 0 and 100.
    • TABLESAMPLE (BUCKET x OUT OF y) :将表格采样到 x 超出 y 分数。TABLESAMPLE(BUCKET x OUT OF y): Sample the table down to a x out of y fraction.

备注

TABLESAMPLE 返回所请求的大约行数或分数。TABLESAMPLE returns the approximate number of rows or fraction requested.

语法Syntax

TABLESAMPLE ({ integer_expression | decimal_expression } PERCENT)
    | TABLESAMPLE ( integer_expression ROWS )
    | TABLESAMPLE ( BUCKET integer_expression OUT OF integer_expression )

示例Examples

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|
+--+----+