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152 lines
4.7 KiB
Markdown
152 lines
4.7 KiB
Markdown
# Conditional functions
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## `if` function
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Syntax: `if(cond, then, else)`
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Returns `then` if the `cond` is truthy(greater than zero), otherwise returns `else`.
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* `cond` must be of type of `UInt8`, and `then` and `else` must have the lowest common type.
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* `then` and `else` can be `NULL`
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**Example:**
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Take this `LEFT_RIGHT` table:
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```sql
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SELECT *
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FROM LEFT_RIGHT
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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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The following query compares `left` and `right` values:
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```sql
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SELECT
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left,
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right,
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if(left < right, 'left is smaller than right', 'right is greater or equal than left') AS is_smaller
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FROM LEFT_RIGHT
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WHERE isNotNull(left) AND isNotNull(right)
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┌─left─┬─right─┬─is_smaller──────────────────────────┐
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│ 1 │ 3 │ left is smaller than right │
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│ 2 │ 2 │ right is greater or equal than left │
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│ 3 │ 1 │ right is greater or equal than left │
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└──────┴───────┴─────────────────────────────────────┘
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```
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Note: `NULL` values are not used in this example, check [NULL values in conditionals](#null-values-in-conditionals) section.
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## Ternary operator
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It works same as `if` function.
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Syntax: `cond ? then : else`
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Returns `then` if the `cond` is truthy(greater than zero), otherwise returns `else`.
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* `cond` must be of type of `UInt8`, and `then` and `else` must have the lowest common type.
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* `then` and `else` can be `NULL`
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## multiIf
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Allows you to write the [CASE](../operators.md#operator_case) operator more compactly in the query.
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Syntax: `multiIf(cond_1, then_1, cond_2, then_2, ..., else)`
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**Parameters:**
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- `cond_N` — The condition for the function to return `then_N`.
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- `then_N` — The result of the function when executed.
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- `else` — The result of the function if none of the conditions is met.
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The function accepts `2N+1` parameters.
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**Returned values**
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The function returns one of the values `then_N` or `else`, depending on the conditions `cond_N`.
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**Example**
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Again using `LEFT_RIGHT` table.
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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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[Original article](https://clickhouse.tech/docs/en/query_language/functions/conditional_functions/) <!--hide-->
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