[WIP] Add if function into ktorm-support-mysql

[hangingman]

Whats' this PR do ?

• Add sum dsl functions into ktorm-support-mysql

  • sumIf: generate sum( if(column condition, ifTrue, ifFalse) )
  • sum: generate sum( column condition )

• In MySQL, sum( boolean ) works as conditional sum

  • ref: Count with IF condition in MySQL query
  • Boolean type in MySQL is represented as INT 0 and 1, just like in C.

use-case

    @Test
    fun testSumIf() {
        val countRich = database.from(Employees)
                .select(sumIf(Employees.salary greaterEq 100L, 1, 0))
                .map { row -> row.getInt(1) }
        assert(countRich.size == 1)
        assert(countRich.first() == 3)
    }

    @Test
    fun testSum() {
        val countRich = database.from(Employees)
                .select(sum(Employees.salary greaterEq 100L))
                .map { row -> row.getInt(1) }
        assert(countRich.size == 1)
        assert(countRich.first() == 3)
    }

[hangingman]

@vincentlauvlwj Hi, I try to add sum and sumIf function in ktorm-support-mysql.
I would appreciate it if you could review this PR.

[vincentlauvlwj]

Please follow the existing code style, fix the indent.

[hangingman]

fixed

[vincentlauvlwj]

Fix the indent.

[vincentlauvlwj]

We can have a overloaded version for Int, fun IF(condition: ColumnDeclaring<Boolean>, then: Int, otherwise: Int), then we don't have to wrap the arguments as ArgumentExpression manually.

Also, overloaded versions for frequent-used types are neccesary, like Int, Long, Double, String, etc.

[hangingman]

@vincentlauvlwj FYI, in this point, at first I created IF function (uppercase).
However, 'detekt' gave me error because of restriction that function name should be started with lower cases.
So, I changed IF to if, and use back-quote.