Sampling queries (Databricks SQL)

The TABLESAMPLE statement is used to sample the table. It supports the following sampling methods:

  • TABLESAMPLE(x ROWS): Sample the table down to the given number of rows.

  • TABLESAMPLE(x PERCENT): Sample the table down to the given percentage. Note that percentages are defined as a number between 0 and 100.

  • TABLESAMPLE(BUCKET x OUT OF y): Sample the table down to a x out of y fraction.

Note

  • TABLESAMPLE returns the approximate number of rows or fraction requested.

  • Always use TABLESAMPLE (X PERCENT) if randomness is important. TABLESAMPLE (X ROWS) is not a simple random sample but instead is implemented using LIMIT.

Syntax 

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

Examples 

> SELECT * FROM test;
  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);
  5 Alex
  2 Mary
  4 Fred
  9 Eric
 10 Adam
  3 Evan

> SELECT * FROM test TABLESAMPLE (5 ROWS);
  5 Alex
  8 Lucy
  2 Mary
  4 Fred
  1 Lisa

> SELECT * FROM test TABLESAMPLE (BUCKET 4 OUT OF 10);
  8 Lucy
  2 Mary
  9 Eric
  6 Mark