ClickHouse/docs/en/sql-reference/functions/functions-for-nulls.md

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/en/sql-reference/functions/functions-for-nulls	135	Nullable

Functions for Working with Nullable Values

isNull

Returns whether the argument is NULL.

See also operator IS NULL.

Syntax

isNull(x)

Alias: ISNULL.

Arguments

• x — A value of non-compound data type.

Returned value

• 1 if x is NULL.
• 0 if x is not NULL.

Example

Table:

┌─x─┬────y─┐
│ 1 │ ᴺᵁᴸᴸ │
│ 2 │    3 │
└───┴──────┘

Query:

SELECT x FROM t_null WHERE isNull(y);

Result:

┌─x─┐
│ 1 │
└───┘

isNullable

Returns 1 if a column is Nullable (i.e allows NULL values), 0 otherwise.

Syntax

isNullable(x)

Arguments

• x — column.

Returned value

• 1 if x allows NULL values. UInt8.
• 0 if x does not allow NULL values. UInt8.

Example

Query:

CREATE TABLE tab (ordinary_col UInt32, nullable_col Nullable(UInt32)) ENGINE = Log;
INSERT INTO tab (ordinary_col, nullable_col) VALUES (1,1), (2, 2), (3,3);
SELECT isNullable(ordinary_col), isNullable(nullable_col) FROM tab;    

Result:

   ┌───isNullable(ordinary_col)──┬───isNullable(nullable_col)──┐
1. │                           0 │                           1 │
2. │                           0 │                           1 │
3. │                           0 │                           1 │
   └─────────────────────────────┴─────────────────────────────┘

isNotNull

Returns whether the argument is not NULL.

See also operator IS NOT NULL.

isNotNull(x)

Arguments:

• x — A value of non-compound data type.

Returned value

• 1 if x is not NULL.
• 0 if x is NULL.

Example

Table:

┌─x─┬────y─┐
│ 1 │ ᴺᵁᴸᴸ │
│ 2 │    3 │
└───┴──────┘

Query:

SELECT x FROM t_null WHERE isNotNull(y);

Result:

┌─x─┐
│ 2 │
└───┘

isNotDistinctFrom

Performs null-safe comparison. Used to compare JOIN keys which contain NULL values in the JOIN ON section. This function will consider two NULL values as identical and will return true, which is distinct from the usual equals behavior where comparing two NULL values would return NULL.

:::note This function is an internal function used by the implementation of JOIN ON. Please do not use it manually in queries. :::

Syntax

isNotDistinctFrom(x, y)

Arguments

• x — first JOIN key.
• y — second JOIN key.

Returned value

• true when x and y are both NULL.
• false otherwise.

Example

For a complete example see: NULL values in JOIN keys.

isZeroOrNull

Returns whether the argument is 0 (zero) or NULL.

isZeroOrNull(x)

Arguments:

• x — A value of non-compound data type.

Returned value

• 1 if x is 0 (zero) or NULL.
• 0 else.

Example

Table:

┌─x─┬────y─┐
│ 1 │ ᴺᵁᴸᴸ │
│ 2 │    0 │
│ 3 │    3 │
└───┴──────┘

Query:

SELECT x FROM t_null WHERE isZeroOrNull(y);

Result:

┌─x─┐
│ 1 │
│ 2 │
└───┘

coalesce

Returns the leftmost non-NULL argument.

coalesce(x,...)

Arguments:

• Any number of parameters of non-compound type. All parameters must be of mutually compatible data types.

Returned values

• The first non-NULL argument
• NULL, if all arguments are NULL.

Example

Consider a list of contacts that may specify multiple ways to contact a customer.

┌─name─────┬─mail─┬─phone─────┬──telegram─┐
│ client 1 │ ᴺᵁᴸᴸ │ 123-45-67 │       123 │
│ client 2 │ ᴺᵁᴸᴸ │ ᴺᵁᴸᴸ      │      ᴺᵁᴸᴸ │
└──────────┴──────┴───────────┴───────────┘

The mail and phone fields are of type String, but the telegram field is UInt32, so it needs to be converted to String.

Get the first available contact method for the customer from the contact list:

SELECT name, coalesce(mail, phone, CAST(telegram,'Nullable(String)')) FROM aBook;

┌─name─────┬─coalesce(mail, phone, CAST(telegram, 'Nullable(String)'))─┐
│ client 1 │ 123-45-67                                                 │
│ client 2 │ ᴺᵁᴸᴸ                                                      │
└──────────┴───────────────────────────────────────────────────────────┘

ifNull

Returns an alternative value if the argument is NULL.

ifNull(x, alt)

Arguments:

• x — The value to check for NULL.
• alt — The value that the function returns if x is NULL.

Returned values

• x if x is not NULL.
• alt if x is NULL.

Example

Query:

SELECT ifNull('a', 'b');

Result:

┌─ifNull('a', 'b')─┐
│ a                │
└──────────────────┘

Query:

SELECT ifNull(NULL, 'b');

Result:

┌─ifNull(NULL, 'b')─┐
│ b                 │
└───────────────────┘

nullIf

Returns NULL if both arguments are equal.

nullIf(x, y)

Arguments:

x, y — Values to compare. Must be of compatible types.

Returned values

• NULL if the arguments are equal.
• x if the arguments are not equal.

Example

Query:

SELECT nullIf(1, 1);

Result:

┌─nullIf(1, 1)─┐
│         ᴺᵁᴸᴸ │
└──────────────┘

Query:

SELECT nullIf(1, 2);

Result:

┌─nullIf(1, 2)─┐
│            1 │
└──────────────┘

assumeNotNull

Returns the corresponding non-Nullable value for a value of Nullable type. If the original value is NULL, an arbitrary result can be returned. See also functions ifNull and coalesce.

assumeNotNull(x)

Arguments:

• x — The original value.

Returned values

• The input value as non-Nullable type, if it is not NULL.
• An arbitrary value, if the input value is NULL.

Example

Table:

┌─x─┬────y─┐
│ 1 │ ᴺᵁᴸᴸ │
│ 2 │    3 │
└───┴──────┘

Query:

SELECT assumeNotNull(y) FROM table;

Result:

┌─assumeNotNull(y)─┐
│                0 │
│                3 │
└──────────────────┘

Query:

SELECT toTypeName(assumeNotNull(y)) FROM t_null;

Result:

┌─toTypeName(assumeNotNull(y))─┐
│ Int8                         │
│ Int8                         │
└──────────────────────────────┘

toNullable

Converts the argument type to Nullable.

toNullable(x)

Arguments:

• x — A value of non-compound type.

Returned value

• The input value but of Nullable type.

Example

Query:

SELECT toTypeName(10);

Result:

┌─toTypeName(10)─┐
│ UInt8          │
└────────────────┘

Query:

SELECT toTypeName(toNullable(10));

Result:

┌─toTypeName(toNullable(10))─┐
│ Nullable(UInt8)            │
└────────────────────────────┘