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Conditional functions
if
Controls conditional branching. Unlike most systems, ClickHouse always evaluate both expressions then
and else
.
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. The type is UInt8, Nullable(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 │
└────────────┘
then
andelse
must have the lowest common type.
Example:
Take this LEFT_RIGHT
table:
SELECT *
FROM LEFT_RIGHT
┌─left─┬─right─┐
│ ᴺᵁᴸᴸ │ 4 │
│ 1 │ 3 │
│ 2 │ 2 │
│ 3 │ 1 │
│ 4 │ ᴺᵁᴸᴸ │
└──────┴───────┘
The following query compares left
and right
values:
SELECT
left,
right,
if(left < right, 'left is smaller than right', 'right is greater or equal than left') AS is_smaller
FROM LEFT_RIGHT
WHERE isNotNull(left) AND isNotNull(right)
┌─left─┬─right─┬─is_smaller──────────────────────────┐
│ 1 │ 3 │ left is smaller than right │
│ 2 │ 2 │ right is greater or equal than left │
│ 3 │ 1 │ right is greater or equal than left │
└──────┴───────┴─────────────────────────────────────┘
Note: NULL
values are not used in this example, check NULL values in conditionals section.
Ternary Operator
It works same as if
function.
Syntax: cond ? then : else
Returns then
if the cond
evaluates to be true (greater than zero), otherwise returns else
.
-
cond
must be of type ofUInt8
, andthen
andelse
must have the lowest common type. -
then
andelse
can beNULL
See also
multiIf
Allows you to write the CASE operator more compactly in the query.
Syntax: multiIf(cond_1, then_1, cond_2, then_2, ..., else)
Parameters:
cond_N
— The condition for the function to returnthen_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
Again using LEFT_RIGHT
table.
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 │
└──────┴───────┴──────────────────┘