Rules for type casting using the operator CAST.

Rules for casting primitive data types.

• When casting primitive data types, some of the source information may be discarded unless contained in the target type. For example:
  • The Float/Double fractional part, when casting to integer types.
  • The Datetime/Timestamp time, when casting to Date.
  • The timezone, when casting from timezone types to date/time types without a timezone.
• If, in a certain combination of the source and target type, casting can't be performed for all possible values of the source type, then, if the casting fails, CAST returns NULL. In such cases, one Optional level is added to the return value type, unless already present. For example, the constructs: CAST("3.14" AS Float?) and CAST("3.14" AS Float) are fully equivalent and return Float?.
• If casting is possible for all values of the source type, then adding '?' works like Just above: CAST(3.14 AS Utf8?) is the same as Just(CAST(3.14 AS Utf8)).
  All primitive type combinations for which CAST is possible are described here.

Casting rules for containers.

Rules for Optional

• If a higher Optional level is set for the target type than for the source type, it's same as adding Just on top of CAST with a lower Optional level.
• If the source type has a higher level of Optional for the source type, then NULL at any level higher than the target level results in NULL.
• At equal levels of Optional, the NULL value preserves the same level.

SELECT
    CAST(1 AS Int32?),                  -- тоже что и Just(1)
    CAST(Just(2/1) AS Float??),         -- [2]
    CAST(Just(3/0) AS Float??) IS NULL; -- false: результат Just(NULL)

Rules for List/Dict

• To create a list, CAST is applied to each item in the source list to cast it to the target type.
• If the target item type is non-optional and CAST on the item might fail, then such casting is discarded. In this case, the resulting list might be shorter or even empty if every casting failed.
• For dictionaries, the casting is totally similar to lists, with CAST being applied to keys and values.

SELECT
    CAST([-1, 0, 1] AS List<Uint8?>),             -- [null, 0, 1]
    CAST(["3.14", "bad", "42"] AS List<Float>),   -- [3.14, 42]

    CAST({-1:3.14, 7:1.6} AS Dict<Uint8, Utf8>),  -- {7: "1.6"}
    CAST({-1:3.14, 7:1.6} AS Dict<Uint8?, Utf8>); -- {7: "1.6", null:"3.14"}

Rules for Struct/Tuple

• A structure or tuple is created by applying CAST to each item of the source type to cast it to an item with the same name or target type index.
• If some field is missing in the target type, it's simply discarded.
• If some field is missing in the source value type, then it can be added only if it's optional and accepts the NULL value.
• If some field is non-optional in the target type, but its casting might fail, then CAST adds Optional to the structure or tuple level and might return NULL for the entire result.

SELECT
    CAST((-1, 0, 1) AS Tuple<Uint16?, Uint16?, Utf8>), -- (null, 0, "1")
    CAST((-2, 0) AS Tuple<Uint16, Utf8>),              -- null
    CAST((3, 4) AS Tuple<Uint16, String>),             -- (3, "4") тип Tuple<Uint16, String>?
    CAST(("4",) AS Tuple<Uint16, String?>),            -- (4, null)
    CAST((5, 6, null) AS Tuple<Uint8?>);               -- (5,)  elements are removed.

SELECT -- One field is removed and one is added: ("three":null, "two": "42")
    CAST(<|one:"8912", two:42|> AS Struct<two:Utf8, three:Date?>).

Rules for Variant

• A variant with a specific name or index is cast to a variant with the same name or index.
• If casting of a variant might fail and the type of this variant is non-optional, then CAST adds Optional to the top level and can return NULL.
• If some variant is missing in the target type, then CAST adds Optional to the top level and returns NULL for such a value.

Nested containers

• All of the above rules are applied recursively for nested containers.