ClickHouse/docs/en/functions/in_functions.rst

Functions for implementing the IN operator
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in, notIn, globalIn, globalNotIn
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See the section "IN operators".

tuple(x, y, ...), operator (x, y, ...)
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A function that allows grouping multiple columns.
For columns with the types T1, T2, ..., it returns a Tuple(T1, T2, ...) type tuple containing these columns. There is no cost to execute the function.
Tuples are normally used as intermediate values for an argument of IN operators, or for creating a list of formal parameters of lambda functions. Tuples can't be written to a table.

tupleElement(tuple, n), operator x.N
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A function that allows getting columns from a tuple.
'N' is the column index, starting from 1. 'N' must be a constant. 'N' must be a strict postive integer no greater than the size of the tuple.
There is no cost to execute the function.