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Merge pull request #48186 from den-crane/patch-61

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Doc. Note about cast and LowCardinality
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Rich Raposa 2023-03-30 08:41:24 -06:00 committed by GitHub
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docs/en/sql-reference/functions/type-conversion-functions.md
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@ -13,6 +13,40 @@ incompatible datatypes (for example from `String` to `Int`). Make sure to check
ClickHouse generally uses the [same behavior as C++ programs](https://en.cppreference.com/w/cpp/language/implicit_conversion).
`to<type>` functions and [cast](#castx-t) have different behaviour in some cases, for example in case of [LowCardinality](../data-types/lowcardinality.md): [cast](#castx-t) removes [LowCardinality](../data-types/lowcardinality.md) trait `to<type>` functions don't. The same with [Nullable](../data-types/nullable.md), this behaviour is not compatible with SQL standard, and it can be changed using [cast_keep_nullable](../../operations/settings/settings.md/#cast_keep_nullable) setting.
Example:
```sql
SELECT
toTypeName(toLowCardinality('') AS val) AS source_type,
toTypeName(toString(val)) AS to_type_result_type,
toTypeName(CAST(val, 'String')) AS cast_result_type
┌─source_type────────────┬─to_type_result_type────┬─cast_result_type─┐
│ LowCardinality(String) │ LowCardinality(String) │ String │
└────────────────────────┴────────────────────────┴──────────────────┘
SELECT
toTypeName(toNullable('') AS val) AS source_type,
toTypeName(toString(val)) AS to_type_result_type,
toTypeName(CAST(val, 'String')) AS cast_result_type
┌─source_type──────┬─to_type_result_type─┬─cast_result_type─┐
│ Nullable(String) │ Nullable(String) │ String │
└──────────────────┴─────────────────────┴──────────────────┘
SELECT
toTypeName(toNullable('') AS val) AS source_type,
toTypeName(toString(val)) AS to_type_result_type,
toTypeName(CAST(val, 'String')) AS cast_result_type
SETTINGS cast_keep_nullable = 1
┌─source_type──────┬─to_type_result_type─┬─cast_result_type─┐
│ Nullable(String) │ Nullable(String) │ Nullable(String) │
└──────────────────┴─────────────────────┴──────────────────┘
```
## toInt(8\|16\|32\|64\|128\|256)
Converts an input value to a value the [Int](/docs/en/sql-reference/data-types/int-uint.md) data type. This function family includes:
@ -994,7 +1028,7 @@ Result:
**See also**
- [cast_keep_nullable](/docs/en/operations/settings/settings.md/#cast_keep_nullable) setting
- [cast_keep_nullable](../../operations/settings/settings.md/#cast_keep_nullable) setting
## accurateCast(x, T)