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6.2 KiB
Functions for working with Nullable aggregates
isNull
Checks whether the argument is NULL.
isNull(x)
Parameters
x
— A value with a non-compound data type.
Returned value
1
ifx
isNULL
.0
ifx
is notNULL
.
Example
Input table
┌─x─┬────y─┐
│ 1 │ ᴺᵁᴸᴸ │
│ 2 │ 3 │
└───┴──────┘
Query
:) SELECT x FROM t_null WHERE isNull(y)
SELECT x
FROM t_null
WHERE isNull(y)
┌─x─┐
│ 1 │
└───┘
1 rows in set. Elapsed: 0.010 sec.
isNotNull
Checks whether the argument is NULL.
isNotNull(x)
Parameters:
x
— A value with a non-compound data type.
Returned value
0
ifx
isNULL
.1
ifx
is notNULL
.
Example
Input table
┌─x─┬────y─┐
│ 1 │ ᴺᵁᴸᴸ │
│ 2 │ 3 │
└───┴──────┘
Query
:) SELECT x FROM t_null WHERE isNotNull(y)
SELECT x
FROM t_null
WHERE isNotNull(y)
┌─x─┐
│ 2 │
└───┘
1 rows in set. Elapsed: 0.010 sec.
coalesce
Checks from left to right whether NULL
arguments were passed and returns the first non-NULL
argument.
coalesce(x,...)
Parameters:
- Any number of parameters of a non-compound type. All parameters must be compatible by data type.
Returned values
- The first non-
NULL
argument. NULL
, if all arguments areNULL
.
Example
Consider a list of contacts that may specify multiple ways to contact a customer.
┌─name─────┬─mail─┬─phone─────┬──icq─┐
│ client 1 │ ᴺᵁᴸᴸ │ 123-45-67 │ 123 │
│ client 2 │ ᴺᵁᴸᴸ │ ᴺᵁᴸᴸ │ ᴺᵁᴸᴸ │
└──────────┴──────┴───────────┴──────┘
The mail
and phone
fields are of type String, but the icq
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 coalesce(mail, phone, CAST(icq,'Nullable(String)')) FROM aBook
SELECT coalesce(mail, phone, CAST(icq, 'Nullable(String)'))
FROM aBook
┌─name─────┬─coalesce(mail, phone, CAST(icq, 'Nullable(String)'))─┐
│ client 1 │ 123-45-67 │
│ client 2 │ ᴺᵁᴸᴸ │
└──────────┴──────────────────────────────────────────────────────┘
2 rows in set. Elapsed: 0.006 sec.
ifNull
Returns an alternative value if the main argument is NULL
.
ifNull(x,alt)
Parameters:
x
— The value to check forNULL
.alt
— The value that the function returns ifx
isNULL
.
Returned values
- The value
x
, ifx
is notNULL
. - The value
alt
, ifx
isNULL
.
Example
SELECT ifNull('a', 'b')
┌─ifNull('a', 'b')─┐
│ a │
└──────────────────┘
SELECT ifNull(NULL, 'b')
┌─ifNull(NULL, 'b')─┐
│ b │
└───────────────────┘
nullIf
Returns NULL
if the arguments are equal.
nullIf(x, y)
Parameters:
x
, y
— Values for comparison. They must be compatible types, or ClickHouse will generate an exception.
Returned values
NULL
, if the arguments are equal.- The
x
value, if the arguments are not equal.
Example
SELECT nullIf(1, 1)
┌─nullIf(1, 1)─┐
│ ᴺᵁᴸᴸ │
└──────────────┘
SELECT nullIf(1, 2)
┌─nullIf(1, 2)─┐
│ 1 │
└──────────────┘
assumeNotNull
Results in a value of type Nullablei for a non- Nullable
, if the value is not NULL
.
assumeNotNull(x)
Parameters:
x
— The original value.
Returned values
- The original value from the non-
Nullable
type, if it is notNULL
. - The default value for the non-
Nullable
type if the original value wasNULL
.
Example
Consider the t_null
table.
SHOW CREATE TABLE t_null
┌─statement─────────────────────────────────────────────────────────────────┐
│ CREATE TABLE default.t_null ( x Int8, y Nullable(Int8)) ENGINE = TinyLog │
└───────────────────────────────────────────────────────────────────────────┘
┌─x─┬────y─┐
│ 1 │ ᴺᵁᴸᴸ │
│ 2 │ 3 │
└───┴──────┘
Apply the resumenotnull
function to the y
column.
SELECT assumeNotNull(y) FROM t_null
┌─assumeNotNull(y)─┐
│ 0 │
│ 3 │
└──────────────────┘
SELECT toTypeName(assumeNotNull(y)) FROM t_null
┌─toTypeName(assumeNotNull(y))─┐
│ Int8 │
│ Int8 │
└──────────────────────────────┘
toNullable
Converts the argument type to Nullable
.
toNullable(x)
Parameters:
x
— The value of any non-compound type.
Returned value
- The input value with a non-
Nullable
type.
Example
SELECT toTypeName(10)
┌─toTypeName(10)─┐
│ UInt8 │
└────────────────┘
SELECT toTypeName(toNullable(10))
┌─toTypeName(toNullable(10))─┐
│ Nullable(UInt8) │
└────────────────────────────┘