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

212 lines
7.1 KiB
Markdown
Raw Normal View History

2020-04-03 13:23:32 +00:00
---
toc_priority: 43
toc_title: 'Conditional '
---
# Conditional Functions {#conditional-functions}
2020-03-18 18:43:51 +00:00
## if {#if}
Controls conditional branching. Unlike most systems, ClickHouse always evaluate both expressions `then` and `else`.
**Syntax**
2020-03-20 10:10:48 +00:00
``` sql
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.
You can use the [short_circuit_function_evaluation](../../operations/settings/settings.md#short-circuit-function-evaluation) setting to calculate the `if` function according to a short scheme. If this setting is enabled, `then` expression is evaluated only on rows where `cond` is true, `else` expression where `cond` is false. For example, an exception about division by zero is not thrown when executing the query `SELECT if(number = 0, 0, intDiv(42, number)) FROM numbers(10)`, because `intDiv(42, number)` will be evaluated only for numbers that doesn't satisfy condition `number = 0`.
**Arguments**
- `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:
2020-03-20 10:10:48 +00:00
``` sql
SELECT if(1, plus(2, 2), plus(2, 6));
```
Result:
2020-03-20 10:10:48 +00:00
``` text
┌─plus(2, 2)─┐
│ 4 │
└────────────┘
```
Query:
2020-03-20 10:10:48 +00:00
``` sql
SELECT if(0, plus(2, 2), plus(2, 6));
```
Result:
2020-03-20 10:10:48 +00:00
``` text
┌─plus(2, 6)─┐
│ 8 │
└────────────┘
```
- `then` and `else` must have the lowest common type.
**Example:**
Take this `LEFT_RIGHT` table:
2020-03-20 10:10:48 +00:00
``` sql
SELECT *
FROM LEFT_RIGHT
┌─left─┬─right─┐
│ ᴺᵁᴸᴸ │ 4 │
│ 1 │ 3 │
│ 2 │ 2 │
│ 3 │ 1 │
│ 4 │ ᴺᵁᴸᴸ │
└──────┴───────┘
```
2020-03-20 10:10:48 +00:00
The following query compares `left` and `right` values:
2020-03-20 10:10:48 +00:00
``` sql
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 │
└──────┴───────┴─────────────────────────────────────┘
```
2020-03-20 10:10:48 +00:00
2020-01-04 20:16:43 +00:00
Note: `NULL` values are not used in this example, check [NULL values in conditionals](#null-values-in-conditionals) section.
2020-03-18 18:43:51 +00:00
## Ternary Operator {#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 of `UInt8`, and `then` and `else` must have the lowest common type.
- `then` and `else` can be `NULL`
**See also**
- [ifNotFinite](../../sql-reference/functions/other-functions.md#ifnotfinite).
2020-03-20 10:10:48 +00:00
## multiIf {#multiif}
Allows you 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)
```
You can use the [short_circuit_function_evaluation](../../operations/settings/settings.md#short-circuit-function-evaluation) setting to calculate the `multiIf` function according to a short scheme. If this setting is enabled, `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, an exception about division by zero is not thrown when executing the query `SELECT multiIf(number = 2, intDiv(1, number), number = 5) FROM numbers(10)`.
**Arguments**
- `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**
Again using `LEFT_RIGHT` table.
2020-03-20 10:10:48 +00:00
``` 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 │
└──────┴───────┴─────────────────┘
```
2020-03-20 10:10:48 +00:00
## Using Conditional Results Directly {#using-conditional-results-directly}
Conditionals always result to `0`, `1` or `NULL`. So you can use conditional results directly like this:
2020-03-20 10:10:48 +00:00
``` sql
SELECT left < right AS is_small
FROM LEFT_RIGHT
┌─is_small─┐
│ ᴺᵁᴸᴸ │
│ 1 │
│ 0 │
│ 0 │
│ ᴺᵁᴸᴸ │
└──────────┘
```
## NULL Values in Conditionals {#null-values-in-conditionals}
When `NULL` values are involved in conditionals, the result will also be `NULL`.
2020-03-20 10:10:48 +00:00
``` 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`.
2020-03-20 10:10:48 +00:00
``` 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 │
└──────┴───────┴──────────────────┘
```