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237 lines
7.9 KiB
Markdown
237 lines
7.9 KiB
Markdown
---
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slug: /en/sql-reference/functions/conditional-functions
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sidebar_position: 40
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sidebar_label: Conditional
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---
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# Conditional Functions
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## if
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Performs conditional branching.
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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.
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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)`.
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`then` and `else` must be of a similar type.
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**Syntax**
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``` sql
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if(cond, then, else)
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```
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Alias: `cond ? then : else` (ternary operator)
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**Arguments**
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- `cond` – The evaluated condition. UInt8, Nullable(UInt8) or NULL.
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- `then` – The expression returned if `condition` is true.
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- `else` – The expression returned if `condition` is `false` or NULL.
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**Returned values**
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The result of either the `then` and `else` expressions, depending on condition `cond`.
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**Example**
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``` sql
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SELECT if(1, plus(2, 2), plus(2, 6));
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```
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Result:
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``` text
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┌─plus(2, 2)─┐
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│ 4 │
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└────────────┘
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```
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## multiIf
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Allows to write the [CASE](../../sql-reference/operators/index.md#operator_case) operator more compactly in the query.
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**Syntax**
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``` sql
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multiIf(cond_1, then_1, cond_2, then_2, ..., else)
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```
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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)`.
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**Arguments**
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The function accepts `2N+1` parameters:
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- `cond_N` — The N-th evaluated condition which controls if `then_N` is returned.
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- `then_N` — The result of the function when `cond_N` is true.
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- `else` — The result of the function if none of conditions is true.
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**Returned values**
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The result of either any of the `then_N` or `else` expressions, depending on the conditions `cond_N`.
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**Example**
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Assuming this table:
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``` text
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┌─left─┬─right─┐
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│ ᴺᵁᴸᴸ │ 4 │
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│ 1 │ 3 │
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│ 2 │ 2 │
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│ 3 │ 1 │
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│ 4 │ ᴺᵁᴸᴸ │
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└──────┴───────┘
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```
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``` sql
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SELECT
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left,
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right,
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multiIf(left < right, 'left is smaller', left > right, 'left is greater', left = right, 'Both equal', 'Null value') AS result
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FROM LEFT_RIGHT
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┌─left─┬─right─┬─result──────────┐
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│ ᴺᵁᴸᴸ │ 4 │ Null value │
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│ 1 │ 3 │ left is smaller │
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│ 2 │ 2 │ Both equal │
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│ 3 │ 1 │ left is greater │
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│ 4 │ ᴺᵁᴸᴸ │ Null value │
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└──────┴───────┴─────────────────┘
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```
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## Using Conditional Results Directly
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Conditionals always result to `0`, `1` or `NULL`. So you can use conditional results directly like this:
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``` sql
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SELECT left < right AS is_small
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FROM LEFT_RIGHT
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┌─is_small─┐
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│ ᴺᵁᴸᴸ │
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│ 1 │
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│ 0 │
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│ 0 │
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│ ᴺᵁᴸᴸ │
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└──────────┘
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```
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## NULL Values in Conditionals
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When `NULL` values are involved in conditionals, the result will also be `NULL`.
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``` sql
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SELECT
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NULL < 1,
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2 < NULL,
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NULL < NULL,
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NULL = NULL
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┌─less(NULL, 1)─┬─less(2, NULL)─┬─less(NULL, NULL)─┬─equals(NULL, NULL)─┐
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│ ᴺᵁᴸᴸ │ ᴺᵁᴸᴸ │ ᴺᵁᴸᴸ │ ᴺᵁᴸᴸ │
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└───────────────┴───────────────┴──────────────────┴────────────────────┘
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```
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So you should construct your queries carefully if the types are `Nullable`.
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The following example demonstrates this by failing to add equals condition to `multiIf`.
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``` sql
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SELECT
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left,
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right,
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multiIf(left < right, 'left is smaller', left > right, 'right is smaller', 'Both equal') AS faulty_result
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FROM LEFT_RIGHT
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┌─left─┬─right─┬─faulty_result────┐
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│ ᴺᵁᴸᴸ │ 4 │ Both equal │
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│ 1 │ 3 │ left is smaller │
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│ 2 │ 2 │ Both equal │
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│ 3 │ 1 │ right is smaller │
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│ 4 │ ᴺᵁᴸᴸ │ Both equal │
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└──────┴───────┴──────────────────┘
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```
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## greatest
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Returns the greatest across a list of values. All of the list members must be of comparable types.
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Examples:
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```sql
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SELECT greatest(1, 2, toUInt8(3), 3.) result, toTypeName(result) type;
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```
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```response
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┌─result─┬─type────┐
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│ 3 │ Float64 │
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└────────┴─────────┘
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```
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:::note
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The type returned is a Float64 as the UInt8 must be promoted to 64 bit for the comparison.
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:::
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```sql
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SELECT greatest(['hello'], ['there'], ['world'])
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```
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```response
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┌─greatest(['hello'], ['there'], ['world'])─┐
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│ ['world'] │
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└───────────────────────────────────────────┘
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```
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```sql
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SELECT greatest(toDateTime32(now() + toIntervalDay(1)), toDateTime64(now(), 3))
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```
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```response
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┌─greatest(toDateTime32(plus(now(), toIntervalDay(1))), toDateTime64(now(), 3))─┐
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│ 2023-05-12 01:16:59.000 │
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└──---──────────────────────────────────────────────────────────────────────────┘
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```
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:::note
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The type returned is a DateTime64 as the DataTime32 must be promoted to 64 bit for the comparison.
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:::
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## least
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Returns the least across a list of values. All of the list members must be of comparable types.
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Examples:
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```sql
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SELECT least(1, 2, toUInt8(3), 3.) result, toTypeName(result) type;
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```
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```response
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┌─result─┬─type────┐
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│ 1 │ Float64 │
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└────────┴─────────┘
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```
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:::note
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The type returned is a Float64 as the UInt8 must be promoted to 64 bit for the comparison.
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:::
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```sql
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SELECT least(['hello'], ['there'], ['world'])
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```
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```response
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┌─least(['hello'], ['there'], ['world'])─┐
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│ ['hello'] │
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└────────────────────────────────────────┘
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```
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```sql
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SELECT least(toDateTime32(now() + toIntervalDay(1)), toDateTime64(now(), 3))
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```
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```response
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┌─least(toDateTime32(plus(now(), toIntervalDay(1))), toDateTime64(now(), 3))─┐
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│ 2023-05-12 01:16:59.000 │
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└────────────────────────────────────────────────────────────────────────────┘
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```
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:::note
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The type returned is a DateTime64 as the DataTime32 must be promoted to 64 bit for the comparison.
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:::
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