mirror of
https://github.com/ClickHouse/ClickHouse.git
synced 2024-12-04 13:32:13 +00:00
267 lines
8.5 KiB
Markdown
267 lines
8.5 KiB
Markdown
---
|
||
slug: /en/sql-reference/functions/conditional-functions
|
||
sidebar_position: 40
|
||
sidebar_label: 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](../../operations/settings/settings.md#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**
|
||
|
||
``` sql
|
||
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**
|
||
|
||
``` sql
|
||
SELECT if(1, plus(2, 2), plus(2, 6));
|
||
```
|
||
|
||
Result:
|
||
|
||
``` text
|
||
┌─plus(2, 2)─┐
|
||
│ 4 │
|
||
└────────────┘
|
||
```
|
||
|
||
## multiIf
|
||
|
||
Allows to write the [CASE](../../sql-reference/operators/index.md#operator_case) operator more compactly in the query.
|
||
|
||
**Syntax**
|
||
|
||
``` sql
|
||
multiIf(cond_1, then_1, cond_2, then_2, ..., else)
|
||
```
|
||
|
||
Setting [short_circuit_function_evaluation](../../operations/settings/settings.md#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:
|
||
|
||
``` text
|
||
┌─left─┬─right─┐
|
||
│ ᴺᵁᴸᴸ │ 4 │
|
||
│ 1 │ 3 │
|
||
│ 2 │ 2 │
|
||
│ 3 │ 1 │
|
||
│ 4 │ ᴺᵁᴸᴸ │
|
||
└──────┴───────┘
|
||
```
|
||
|
||
``` sql
|
||
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:
|
||
|
||
``` sql
|
||
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`.
|
||
|
||
``` sql
|
||
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`.
|
||
|
||
``` sql
|
||
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 │
|
||
└──────┴───────┴──────────────────┘
|
||
```
|
||
|
||
## greatest
|
||
|
||
Returns the greatest across a list of values. All of the list members must be of comparable types.
|
||
|
||
Examples:
|
||
|
||
```sql
|
||
SELECT greatest(1, 2, toUInt8(3), 3.) result, toTypeName(result) type;
|
||
```
|
||
```response
|
||
┌─result─┬─type────┐
|
||
│ 3 │ Float64 │
|
||
└────────┴─────────┘
|
||
```
|
||
|
||
:::note
|
||
The type returned is a Float64 as the UInt8 must be promoted to 64 bit for the comparison.
|
||
:::
|
||
|
||
```sql
|
||
SELECT greatest(['hello'], ['there'], ['world'])
|
||
```
|
||
```response
|
||
┌─greatest(['hello'], ['there'], ['world'])─┐
|
||
│ ['world'] │
|
||
└───────────────────────────────────────────┘
|
||
```
|
||
|
||
```sql
|
||
SELECT greatest(toDateTime32(now() + toIntervalDay(1)), toDateTime64(now(), 3))
|
||
```
|
||
```response
|
||
┌─greatest(toDateTime32(plus(now(), toIntervalDay(1))), toDateTime64(now(), 3))─┐
|
||
│ 2023-05-12 01:16:59.000 │
|
||
└──---──────────────────────────────────────────────────────────────────────────┘
|
||
```
|
||
|
||
:::note
|
||
The type returned is a DateTime64 as the DataTime32 must be promoted to 64 bit for the comparison.
|
||
:::
|
||
|
||
## least
|
||
|
||
Returns the least across a list of values. All of the list members must be of comparable types.
|
||
|
||
Examples:
|
||
|
||
```sql
|
||
SELECT least(1, 2, toUInt8(3), 3.) result, toTypeName(result) type;
|
||
```
|
||
```response
|
||
┌─result─┬─type────┐
|
||
│ 1 │ Float64 │
|
||
└────────┴─────────┘
|
||
```
|
||
|
||
:::note
|
||
The type returned is a Float64 as the UInt8 must be promoted to 64 bit for the comparison.
|
||
:::
|
||
|
||
```sql
|
||
SELECT least(['hello'], ['there'], ['world'])
|
||
```
|
||
```response
|
||
┌─least(['hello'], ['there'], ['world'])─┐
|
||
│ ['hello'] │
|
||
└────────────────────────────────────────┘
|
||
```
|
||
|
||
```sql
|
||
SELECT least(toDateTime32(now() + toIntervalDay(1)), toDateTime64(now(), 3))
|
||
```
|
||
```response
|
||
┌─least(toDateTime32(plus(now(), toIntervalDay(1))), toDateTime64(now(), 3))─┐
|
||
│ 2023-05-12 01:16:59.000 │
|
||
└────────────────────────────────────────────────────────────────────────────┘
|
||
```
|
||
|
||
:::note
|
||
The type returned is a DateTime64 as the DataTime32 must be promoted to 64 bit for the comparison.
|
||
:::
|
||
|
||
## clamp
|
||
|
||
Constrain the return value between A and B.
|
||
|
||
**Syntax**
|
||
|
||
``` sql
|
||
clamp(x, min, max)
|
||
```
|
||
|
||
**Arguments**
|
||
|
||
- `x` – Input value.
|
||
- `min` – Limit the lower bound.
|
||
- `max` – Limit the upper bound.
|
||
|
||
**Returned values**
|
||
|
||
If the value is less than the minimum value, return the minimum value; if it is greater than the maximum value, return the maximum value; otherwise, return the current value.
|
||
|
||
Examples:
|
||
|
||
```sql
|
||
SELECT clamp(1, 2, 3) result, toTypeName(result) type;
|
||
```
|
||
```response
|
||
┌─result─┬─type────┐
|
||
│ 2 │ Float64 │
|
||
└────────┴─────────┘
|
||
``` |