ClickHouse/docs/en/sql-reference/aggregate-functions/reference/uniq.md

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uniq

Calculates the approximate number of different values of the argument.

uniq(x[, ...])

Arguments

The function takes a variable number of parameters. Parameters can be Tuple, Array, Date, DateTime, String, or numeric types.

Returned value

• A UInt64-type number.

Implementation details

Function:

• Calculates a hash for all parameters in the aggregate, then uses it in calculations.

• Uses an adaptive sampling algorithm. For the calculation state, the function uses a sample of element hash values up to 65536.

  This algorithm is very accurate and very efficient on the CPU. When the query contains several of these functions, using `uniq` is almost as fast as using other aggregate functions.
  

• Provides the result deterministically (it doesn’t depend on the query processing order).

We recommend using this function in almost all scenarios.

See Also

• uniqCombined
• uniqCombined64
• uniqHLL12
• uniqExact