feat: EXPOSED-85 Add support for changing default value in SQL Server

[joc-a]

When creating a column with a DEFAULT value in SQL Server, a DEFAULT constraint is created under the hood. To modify the default value of a column, the constraint has to be dropped first. To do that, the name of the constraint is needed. To be able to obtain its name easily, the way the DEFAULT constraint is created was modified to give it an explicit name with the format DF_TableName_ColumnName. So when adding a new DEFAULT value to a column or modifying an existing one, it's necessary to drop the existing DEFAULT constraint first (if it exists), and then add a new one.

Every alter statement will be a separate item in the list returned by the modifyColumn function. The statement to alter the default value of a column will combine both the DROP CONSTRAINT and ADD CONSTRAINT statements in one String item, separated by a semicolon. The statement to alter the type and nullability will be part of one String item.

Example:

val t1 = object : Table("foo") {
    val id = integer("idcol")
    val col = integer("col")
    override val primaryKey = PrimaryKey(id)
}
val t2 = object : Table("foo") {
    val id = integer("idcol")
    val col = integer("COL").nullable().default(1)
    override val primaryKey = PrimaryKey(id)
}

withDb(excludeSettings = listOf(TestDB.SQLITE)) {
    try {
        SchemaUtils.createMissingTablesAndColumns(t1)
        val missingStatements = SchemaUtils.addMissingColumnsStatements(t2)
        println(missingStatements)
    } finally {
        SchemaUtils.drop(t1)
    }
}

Expected output: [ALTER TABLE foo ALTER COLUMN COL INT NULL, ALTER TABLE foo DROP CONSTRAINT IF EXISTS DF_foo_COL; ALTER TABLE foo ADD CONSTRAINT DF_foo_COL DEFAULT 1 for COL]

It's possible to put everything in a single String item without any commas or semicolons. This works:
exec("ALTER TABLE foo ALTER COLUMN COL INT NULL ALTER TABLE foo DROP CONSTRAINT IF EXISTS DF_foo_COL ALTER TABLE foo ADD CONSTRAINT DF_foo_COL DEFAULT 1 for COL")
But I thought it would be neater to separate them.

[bog-walk]

@joc-a The test file UnsignedColumnTypeTests uses Column.modifyStatement() a few times (which in turn uses modifyColumn(this, ColumnDiff.AllChanged)).

Could you confirm if those tests are now failing on SQL Server? The tests that use it may need to replace it with a raw exec("""ALTER TABLE ... ALTER COLUMN""").

[e5l]

lgtm

[joc-a]

This logic was copied from Column.descriptionDdl function, because this is what modifyColumn in SQLServerDialect was using before, when it was invoking super.modifyColumn:

override fun modifyColumn(column: Column<*>, columnDiff: ColumnDiff): List<String> =
        listOf("ALTER TABLE ${TransactionManager.current().identity(column.table)} MODIFY COLUMN ${column.descriptionDdl(true)}")