[CALCITE-6149] Unparse for CAST Nullable with ClickHouseSqlDialect

[Chen768959]

In ClickHouse, there is an issue with RelDataType of Nullable type when casting to a non-Nullable type. For example, SELECT CAST(NULL, 'Int32') will throw an error. In such cases, we should use SELECT CAST(NULL, 'Nullable(Int32)') instead.

CREATE TABLE default.test
(
    `id` Int64,
    `a` Nullable(Int64)
)
ENGINE = MergeTree
ORDER BY id

insert into test(id,a) values(1,1);
insert into test(id,a) values(2,null);
SELECT CAST(a, 'String') FROM test;

DB::Exception: Cannot convert NULL value to non-Nullable type

[JiajunBernoulli]

Do you have not-null case?

[Chen768959]

Okay, let me add these case.

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
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0 Security Hotspots
0 Code Smells

53.3% Coverage
0.0% Duplication

[mihaibudiu]

If this is ready for merging can you please rebase and resolve the conflicts?
Thank you.

[Chen768959]

If this is ready for merging can you please rebase and resolve the conflicts?
Thank you.

Sure