Iss 507 support filter clause

[ahmad-moussawi]

This PR adds the support for the SelectAggregate method and it's siblings SelectSum, SelectMax, SelectMin, SelectAvg, SelectCount methods, this is better than using SelectRaw.

All these methods accepts an optional filter parameter to filter the aggregated column based on a specified conditions, this will translate to the Aggregation Filter clause on supported compilers like PostgreSQL, and SQLite with a fallback to CASE WHEN statement for the unsupported ones.

Related #507

[yankyhgoflow]

👍 Thanks, this is really taking this library to the next level!

Thanks @ahmad-moussawi you rock!

[kenzouno1]

Missing alias for this feature

[ahmad-moussawi]

@kenzouno1 what do you mean exactly by missing alias? is this what you are looking for?

var query = db.Query("Transaction")
                .SelectSum("Amount as Total_2020", q => q.WhereDatePart("year", "date", 2020))
                .SelectSum("Amount as Total_2021", q => q.WhereDatePart("year", "date", 2021))
                .SelectSum("Amount as Total_2022", q => q.WhereDatePart("year", "date", 2022))

check more examples here
https://github.com/sqlkata/querybuilder/blob/master/QueryBuilder.Tests/MySqlExecutionTest.cs#L237

[kenzouno1]

That's exactly what I'm looking for. Thank you