دوال للتعامل مع القيم Nullable

assumeNotNull(x)

CREATE TABLE t_null (x Int8, y Nullable(Int8))
ENGINE=MergeTree()
ORDER BY x;

INSERT INTO t_null VALUES (1, NULL), (2, 3);

SELECT assumeNotNull(y) FROM t_null;
SELECT toTypeName(assumeNotNull(y)) FROM t_null;

┌─assumeNotNull(y)─┐
│                0 │
│                3 │
└──────────────────┘
┌─toTypeName(assumeNotNull(y))─┐
│ Int8                         │
│ Int8                         │
└──────────────────────────────┘

coalesce(x[, y, ...])

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

CREATE TABLE aBook
(
    name String,
    mail Nullable(String),
    phone Nullable(String),
    telegram Nullable(UInt32)
)
ENGINE = MergeTree
ORDER BY tuple();

INSERT INTO aBook VALUES ('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                                                      │
└──────────┴───────────────────────────────────────────────────────────┘

firstNonDefault(arg1[, arg2[ ...]])

SELECT firstNonDefault(0, 1, 2)

1

SELECT firstNonDefault('', 'hello', 'world')

hello

SELECT firstNonDefault(NULL, 0 :: UInt8, 1 :: UInt8)

1

SELECT firstNonDefault(NULL, 0 :: Nullable(UInt8), 1 :: Nullable(UInt8))

0

ifNull(x, alt)

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

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

isNotNull(x)

CREATE TABLE t_null
(
  x Int32,
  y Nullable(Int32)
)
ENGINE = MergeTree
ORDER BY tuple();

INSERT INTO t_null VALUES (1, NULL), (2, 3);

SELECT x FROM t_null WHERE isNotNull(y);

┌─x─┐
│ 2 │
└───┘

isNull(x)

CREATE TABLE t_null
(
  x Int32,
  y Nullable(Int32)
)
ENGINE = MergeTree
ORDER BY tuple();

INSERT INTO t_null VALUES (1, NULL), (2, 3);

SELECT x FROM t_null WHERE isNull(y);

┌─x─┐
│ 1 │
└───┘

isNullable(x)

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

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

isZeroOrNull(x)

CREATE TABLE t_null
(
  x Int32,
  y Nullable(Int32)
)
ENGINE = MergeTree
ORDER BY tuple();

INSERT INTO t_null VALUES (1, NULL), (2, 0), (3, 3);

SELECT x FROM t_null WHERE isZeroOrNull(y);

┌─x─┐
│ 1 │
│ 2 │
└───┘

nullIf(x, y)

SELECT nullIf(1, 1), nullIf(1, 2);

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

toNullable(x)

SELECT toTypeName(10), toTypeName(toNullable(10));

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