Relational Data - Data Types - Enum

From FojiSoft Docs

Enumerated type consisting of named values.

Named values can be declared as 'string' = integer pairs or 'string' names . ClickHouse stores only numbers, but supports operations with the values through their names.

ClickHouse supports:

  • 8-bit Enum. It can contain up to 256 values enumerated in the [-128, 127] range.
  • 16-bit Enum. It can contain up to 65536 values enumerated in the [-32768, 32767] range.

ClickHouse automatically chooses the type of Enum when data is inserted. You can also use Enum8 or Enum16 types to be sure in the size of storage.

Usage Examples

Here we create a table with an Enum8('hello' = 1, 'world' = 2) type column: 

CREATE TABLE t_enum
(
    x Enum('hello' = 1, 'world' = 2)
)
ENGINE = TinyLog

Similarly, you could omit numbers. ClickHouse will assign consecutive numbers automatically. Numbers are assigned starting from 1 by default. 

CREATE TABLE t_enum
(
    x Enum('hello', 'world')
)
ENGINE = TinyLog

You can also specify legal starting number for the first name. 

CREATE TABLE t_enum
(
    x Enum('hello' = 1, 'world')
)
ENGINE = TinyLog
CREATE TABLE t_enum
(
    x Enum8('hello' = -129, 'world')
)
ENGINE = TinyLog
Exception on server:
Code: 69. DB::Exception: Value -129 for element 'hello' exceeds range of Enum8.

Column x can only store values that are listed in the type definition: 'hello' or 'world'. If you try to save any other value, ClickHouse will raise an exception. 8-bit size for this Enum is chosen automatically. 

INSERT INTO t_enum VALUES ('hello'), ('world'), ('hello')
Ok.
INSERT INTO t_enum values('a')
Exception on client:
Code: 49. DB::Exception: Unknown element 'a' for type Enum('hello' = 1, 'world' = 2)

When you query data from the table, ClickHouse outputs the string values from Enum. 

SELECT * FROM t_enum
┌─x─────┐
│ hello │
│ world │
│ hello │
└───────┘

If you need to see the numeric equivalents of the rows, you must cast the Enum value to integer type. 

SELECT CAST(x, 'Int8') FROM t_enum
┌─CAST(x, 'Int8')─┐
│               1 │
│               2 │
│               1 │
└─────────────────┘

To create an Enum value in a query, you also need to use CAST. 

SELECT toTypeName(CAST('a', 'Enum(\'a\' = 1, \'b\' = 2)'))
┌─toTypeName(CAST('a', 'Enum(\'a\' = 1, \'b\' = 2)'))─┐
│ Enum8('a' = 1, 'b' = 2)                             │
└─────────────────────────────────────────────────────┘

General Rules and Usage

Each of the values is assigned a number in the range -128 ... 127 for Enum8 or in the range -32768 ... 32767 for Enum16. All the strings and numbers must be different. An empty string is allowed. If this type is specified (in a table definition), numbers can be in an arbitrary order. However, the order does not matter.

Neither the string nor the numeric value in an Enum can be NULL.

An Enum can be contained in Nullable type. So if you create a table using the query 

CREATE TABLE t_enum_nullable
(
    x Nullable( Enum8('hello' = 1, 'world' = 2) )
)
ENGINE = TinyLog

it can store not only 'hello' and 'world', but NULL, as well. 

INSERT INTO t_enum_nullable Values('hello'),('world'),(NULL)

In RAM, an Enum column is stored in the same way as Int8 or Int16 of the corresponding numerical values.

When reading in text form, ClickHouse parses the value as a string and searches for the corresponding string from the set of Enum values. If it is not found, an exception is thrown. When reading in text format, the string is read and the corresponding numeric value is looked up. An exception will be thrown if it is not found. When writing in text form, it writes the value as the corresponding string. If column data contains garbage (numbers that are not from the valid set), an exception is thrown. When reading and writing in binary form, it works the same way as for Int8 and Int16 data types. The implicit default value is the value with the lowest number.

During ORDER BY, GROUP BY, IN, DISTINCT and so on, Enums behave the same way as the corresponding numbers. For example, ORDER BY sorts them numerically. Equality and comparison operators work the same way on Enums as they do on the underlying numeric values.

Enum values cannot be compared with numbers. Enums can be compared to a constant string. If the string compared to is not a valid value for the Enum, an exception will be thrown. The IN operator is supported with the Enum on the left-hand side and a set of strings on the right-hand side. The strings are the values of the corresponding Enum.

Most numeric and string operations are not defined for Enum values, e.g. adding a number to an Enum or concatenating a string to an Enum. However, the Enum has a natural toString function that returns its string value.

Enum values are also convertible to numeric types using the toT function, where T is a numeric type. When T corresponds to the enum’s underlying numeric type, this conversion is zero-cost. The Enum type can be changed without cost using ALTER, if only the set of values is changed. It is possible to both add and remove members of the Enum using ALTER (removing is safe only if the removed value has never been used in the table). As a safeguard, changing the numeric value of a previously defined Enum member will throw an exception.

Using ALTER, it is possible to change an Enum8 to an Enum16 or vice versa, just like changing an Int8 to Int16.

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