ClickHouse/docs/en/sql-reference/functions/conditional-functions.md

slug	sidebar_position	sidebar_label
/en/sql-reference/functions/conditional-functions	40	Conditional

Conditional Functions

if

Performs conditional branching.

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

Setting short_circuit_function_evaluation controls whether short-circuit evaluation is used. If enabled, the then expression is evaluated only on rows where cond is true and the else expression where cond is false. For example, with short-circuit evaluation, no division-by-zero exception is thrown when executing the query SELECT if(number = 0, 0, intDiv(42, number)) FROM numbers(10).

then and else must be of a similar type.

Syntax

if(cond, then, else)

Alias: cond ? then : else (ternary operator)

Arguments

• cond – The evaluated condition. UInt8, Nullable(UInt8) or NULL.
• then – The expression returned if condition is true.
• else – The expression returned if condition is false or NULL.

Returned values

The result of either the then and else expressions, depending on condition cond.

Example

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

Result:

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

multiIf

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

Syntax

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

Setting short_circuit_function_evaluation controls whether short-circuit evaluation is used. If enabled, the then_i expression is evaluated only on rows where ((NOT cond_1) AND (NOT cond_2) AND ... AND (NOT cond_{i-1}) AND cond_i) is true, cond_i will be evaluated only on rows where ((NOT cond_1) AND (NOT cond_2) AND ... AND (NOT cond_{i-1})) is true. For example, with short-circuit evaluation, no division-by-zero exception is thrown when executing the query SELECT multiIf(number = 2, intDiv(1, number), number = 5) FROM numbers(10).

Arguments

The function accepts 2N+1 parameters:

• cond_N — The N-th evaluated condition which controls if then_N is returned.
• then_N — The result of the function when cond_N is true.
• else — The result of the function if none of conditions is true.

Returned values

The result of either any of the then_N or else expressions, depending on the conditions cond_N.

Example

Assuming this table:

┌─left─┬─right─┐
│ ᴺᵁᴸᴸ │     4 │
│    1 │     3 │
│    2 │     2 │
│    3 │     1 │
│    4 │  ᴺᵁᴸᴸ │
└──────┴───────┘

SELECT
    left,
    right,
    multiIf(left < right, 'left is smaller', left > right, 'left is greater', left = right, 'Both equal', 'Null value') AS result
FROM LEFT_RIGHT

┌─left─┬─right─┬─result──────────┐
│ ᴺᵁᴸᴸ │     4 │ Null value      │
│    1 │     3 │ left is smaller │
│    2 │     2 │ Both equal      │
│    3 │     1 │ left is greater │
│    4 │  ᴺᵁᴸᴸ │ Null value      │
└──────┴───────┴─────────────────┘

Using Conditional Results Directly

Conditionals always result to 0, 1 or NULL. So you can use conditional results directly like this:

SELECT left < right AS is_small
FROM LEFT_RIGHT

┌─is_small─┐
│     ᴺᵁᴸᴸ │
│        1 │
│        0 │
│        0 │
│     ᴺᵁᴸᴸ │
└──────────┘

NULL Values in Conditionals

When NULL values are involved in conditionals, the result will also be NULL.

SELECT
    NULL < 1,
    2 < NULL,
    NULL < NULL,
    NULL = NULL

┌─less(NULL, 1)─┬─less(2, NULL)─┬─less(NULL, NULL)─┬─equals(NULL, NULL)─┐
│ ᴺᵁᴸᴸ          │ ᴺᵁᴸᴸ          │ ᴺᵁᴸᴸ             │ ᴺᵁᴸᴸ               │
└───────────────┴───────────────┴──────────────────┴────────────────────┘

So you should construct your queries carefully if the types are Nullable.

The following example demonstrates this by failing to add equals condition to multiIf.

SELECT
    left,
    right,
    multiIf(left < right, 'left is smaller', left > right, 'right is smaller', 'Both equal') AS faulty_result
FROM LEFT_RIGHT

┌─left─┬─right─┬─faulty_result────┐
│ ᴺᵁᴸᴸ │     4 │ Both equal       │
│    1 │     3 │ left is smaller  │
│    2 │     2 │ Both equal       │
│    3 │     1 │ right is smaller │
│    4 │  ᴺᵁᴸᴸ │ Both equal       │
└──────┴───────┴──────────────────┘