取樣查詢Sampling queries

TABLESAMPLE語句是用來範例資料表。The TABLESAMPLE statement is used to sample the table. 它支援下列取樣方法:It supports the following sampling methods:

  • TABLESAMPLE (x ROWS) :將資料表取樣至指定的資料列數目。TABLESAMPLE(x ROWS): Sample the table down to the given number of rows.
  • TABLESAMPLE (x PERCENT) :將資料表取樣至指定的百分比。TABLESAMPLE(x PERCENT): Sample the table down to the given percentage. 請注意,百分比會定義為介於0和100之間的數位。Note that percentages are defined as a number between 0 and 100.
  • TABLESAMPLE (BUCKET x OUT OF y) :將資料表取樣為一 x y 小部分。TABLESAMPLE(BUCKET x OUT OF y): Sample the table down to a x out of y fraction.

注意

TABLESAMPLE 傳回所要求的資料列或分數的大約數目。TABLESAMPLE returns the approximate number of rows or fraction requested.

語法Syntax

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

範例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|
+--+----+