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

Conditional functions

if

Controls conditional branching. Unlike most systems, ClickHouse considers the condition cond and expressions then and else. This is needed to consider answer quickly.

Syntax

SELECT if(cond, then, else)

If the condition cond evaluates to a non-zero value, returns the result of the expression then, and the result of the expression else, if present, is skipped. If the cond is zero or NULL, then the result of the then expression is skipped and the result of the else expression, if present, is returned.

Parameters

• cond – The condition for evaluation that can be zero or not. Can be UInt8 or NULL.
• then - The expression to return if condition is met.
• else - The expression to return if condition is not met.

Returned values

The function executes then and else expressions and returns its result, depending on whether the condition cond ended up being zero or not.

Example

Query:

SELECT if(1, plus(2, 2), plus(2, 6))

Result:

┌─plus(2, 2)─┐
│          4 │
└────────────┘

Query:

SELECT if(0, plus(2, 2), plus(2, 6))

Result:

┌─plus(2, 6)─┐
│          8 │
└────────────┘

multiIf

Allows you to write the CASE operator more compactly in the query.

multiIf(cond_1, then_1, cond_2, then_2...else)

Parameters:

• cond_N — The condition for the function to return then_N.
• then_N — The result of the function when executed.
• else — The result of the function if none of the conditions is met.

The function accepts 2N+1 parameters.

Returned values

The function returns one of the values then_N or else, depending on the conditions cond_N.

Example

Take the table

┌─x─┬────y─┐
│ 1 │ ᴺᵁᴸᴸ │
│ 2 │    3 │
└───┴──────┘

Run the query SELECT multiIf(isNull(y) x, y < 3, y, NULL) FROM t_null. Result:

┌─multiIf(isNull(y), x, less(y, 3), y, NULL)─┐
│                                          1 │
│                                       ᴺᵁᴸᴸ │
└────────────────────────────────────────────┘

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