Function registerDataTypeMap

src/DataTypes/DataTypeMap.cpp:251–490

void registerDataTypeMap(DataTypeFactory & factory)
{
    factory.registerDataType("Map", create, DataTypeFactory::Case::Sensitive, Documentation{
            .description = R"DOCS_MD(
Data type `Map(K, V)` stores key-value pairs.

Unlike other databases, maps are not unique in ClickHouse, i.e. a map can contain two elements with the same key.
(The reason for that is that maps are internally implemented as `Array(Tuple(K, V))`.)

You can use use syntax `m[k]` to obtain the value for key `k` in map `m`.
Also, `m[k]` scans the map, i.e. the runtime of the operation is linear in the size of the map.

**Parameters**

- `K` — The type of the Map keys. Arbitrary type except [Nullable](../../sql-reference/data-types/nullable.md) and [LowCardinality](../../sql-reference/data-types/lowcardinality.md) nested with [Nullable](../../sql-reference/data-types/nullable.md) types.
- `V` — The type of the Map values. Arbitrary type.

**Examples**

Create a table with a column of type map:

```sql title="Query"
CREATE TABLE tab (m Map(String, UInt64)) ENGINE=Memory;
INSERT INTO tab VALUES ({'key1':1, 'key2':10}), ({'key1':2,'key2':20}), ({'key1':3,'key2':30});
```

To select `key2` values:

```sql title="Query"
SELECT m['key2'] FROM tab;
```

```text title="Response"
┌─arrayElement(m, 'key2')─┐
│                      10 │
│                      20 │
│                      30 │
└─────────────────────────┘
```

If the requested key `k` is not contained in the map, `m[k]` returns the value type's default value, e.g. `0` for integer types and `''` for string types.
To check whether a key exists in a map, you can use function [mapContains](/sql-reference/functions/tuple-map-functions#mapContainsKey).

```sql title="Query"
CREATE TABLE tab (m Map(String, UInt64)) ENGINE=Memory;
INSERT INTO tab VALUES ({'key1':100}), ({});
SELECT m['key1'] FROM tab;
```

```text title="Response"
┌─arrayElement(m, 'key1')─┐
│                     100 │
│                       0 │
└─────────────────────────┘
```

## Converting Tuple to Map {#converting-tuple-to-map}