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56 lines
1.8 KiB
Markdown
56 lines
1.8 KiB
Markdown
<a name="data_type-nullable"></a>
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# Nullable(TypeName)
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Allows to store special marker ([NULL](../query_language/syntax.md#null-literal)) that denotes "missing value" alongside normal values allowed by `TypeName`. For example, a `Nullable(Int8)` type column can store `Int8` type values, and the rows that don't have a value will store `NULL`.
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For a `TypeName`, you can't use composite data types [Array](array.md#data_type is array) and [Tuple](tuple.md#data_type-tuple). Composite data types can contain `Nullable` type values, such as `Array(Nullable(Int8))`.
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A `Nullable` type field can't be included in table indexes.
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`NULL` is the default value for any `Nullable` type, unless specified otherwise in the ClickHouse server configuration.
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## Storage features
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To store `Nullable` type values in table column, ClickHouse uses a separate file with `NULL` masks in addition to normal file with values. Entries in masks file allow ClickHouse to distinguish between `NULL` and default value of corresponding data type for each table row. Because of additional file, `Nullable` column consumes additional storage space compared to similar normal one.
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!!! info "Note"
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Using `Nullable` almost always negatively affects performance, keep this in mind when designing your databases.
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## Usage example
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```
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:) CREATE TABLE t_null(x Int8, y Nullable(Int8)) ENGINE TinyLog
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CREATE TABLE t_null
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(
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x Int8,
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y Nullable(Int8)
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)
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ENGINE = TinyLog
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Ok.
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0 rows in set. Elapsed: 0.012 sec.
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:) INSERT INTO t_null VALUES (1, NULL)
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INSERT INTO t_null VALUES
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Ok.
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1 rows in set. Elapsed: 0.007 sec.
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:) SELECT x + y FROM t_null
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SELECT x + y
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FROM t_null
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┌─plus(x, y)─┐
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│ ᴺᵁᴸᴸ │
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│ 5 │
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└────────────┘
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2 rows in set. Elapsed: 0.144 sec.
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```
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