ClickHouse/docs/en/query_language/functions/in_functions.md

Functions for implementing the IN operator

in, notIn, globalIn, globalNotIn

See the section IN operators.

tuple(x, y, ...), operator (x, y, ...)

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

A function that allows getting a column from a tuple. 'N' is the column index, starting from 1. N must be a constant. '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.

Original article