dialect/sql/schema: initial work for incremental migration (#428)

This is a WIP PR and should be ignored this moment.
It's based on PR #221 created by Erik Hollensbe (He should
get his credit for his work before we land this).

dialect/sql/schema/mysql.go

@@ -243,11 +243,6 @@ func (d *MySQL) addColumn(c *Column) *sql.ColumnBuilder {
 	return b
 }
 
-// alterColumn returns the DSL query for modifying the given column.
-func (d *MySQL) alterColumn(c *Column) []*sql.ColumnBuilder {
-	return []*sql.ColumnBuilder{d.addColumn(c)}
-}
-
 // addIndex returns the querying for adding an index to MySQL.
 func (d *MySQL) addIndex(i *Index, table string) *sql.IndexBuilder {
 	return i.Builder(table)
@@ -465,6 +460,24 @@ func (d *MySQL) tableSchema() sql.Querier {
 	return sql.Raw("(SELECT DATABASE())")
 }
 
+// alterColumns returns the queries for applying the columns change-set.
+func (d *MySQL) alterColumns(table string, add, modify, drop []*Column) sql.Queries {
+	b := sql.Dialect(dialect.MySQL).AlterTable(table)
+	for _, c := range add {
+		b.AddColumn(d.addColumn(c))
+	}
+	for _, c := range modify {
+		b.ModifyColumn(d.addColumn(c))
+	}
+	for _, c := range drop {
+		b.DropColumn(sql.Dialect(dialect.MySQL).Column(c.Name))
+	}
+	if len(b.Queries) == 0 {
+		return nil
+	}
+	return sql.Queries{b}
+}
+
 // parseColumn returns column parts, size and signedness by mysql type
 func parseColumn(typ string) (parts []string, size int64, unsigned bool, err error) {
 	switch parts = strings.FieldsFunc(typ, func(r rune) bool {