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DataType conversion

Explicit Casting​

We have two kinds of expression to cast a value to another datatype.

  1. CAST function, if error happens during cast, it throws error.

We also support pg casting style: CAST(c as INT) is same as c::Int

  1. TRY_CAST function if error happens during cast, it returns NULL.

Implicit Casting ("Coercion")​

Some basic rules about "Coercion" aka (Auto casting)

  1. All integer datatypes can be implicitly casted into BIGINT aka (INT64) datatype.

e.g.

Int --> bigint
UInt8 --> bigint
Int32 --> bigint
  1. All numeric datatypes can be implicitly casted into Double aka (Float64) datatype.

e.g.

Int --> Double
Float --> Double
Int32 --> Double
  1. ALL non-nullable datatypes T can be implicitly casted into Nullable(T) datatype.

e.g.

Int --> Nullable<Int>
String -->  Nullable<String>
  1. All datatypes can be implicitly casted into Variant datatype.

e.g.

Int --> Variant
  1. String datatype is the lowest datatype that can't be implicitly casted to other datatypes.
  2. Array<T> --> Array<U> if T --> U.
  3. Nullable<T> --> Nullable<U> if T--> U.
  4. Null --> Nullable<T> for any T datatype.
  5. Numberic can be implicitly casted to other numeric datatype if there is no lossy.

FAQ​

Why can't the numeric type be automatically converted to the String type.

It's trivial and even works in other popular databases. But it'll introduce ambiguity.

e.g.

select 39 > '301';
select 39 = '  39  ';

We don't know how to compare them with numberic rules or String rules. Because they are different result accoring to different rules.

select 39 > 301 is false where select '39' > '301' is true.

To make the syntax more precise and less ambiguous, we throw the error to user and get more precise SQL.

Why can't the boolean type be automatically converted to the numeric type.

This will also bring ambiguity. e.g.

select true > 0.5;

What's the error message: "can't cast from nullable data into non-nullable type".

That means you got a null in your source column. You can use TRY_CAST function or make your target type be a nullable type.

select concat(1, col) not work

You can improve the SQL to select concat('1', col).

We may improve the expression in the future which could parse the literal 1 into String value if possible (the concat function just accpet String parameters).