Add aggregation function to select query builder (#39)

qb: added min, max, avg, sum, and count to SelectBuilder

qb/select.go

@@ -163,3 +163,44 @@ func (b *SelectBuilder) AllowFiltering() *SelectBuilder {
 	b.allowFiltering = true
 	return b
 }
+
+// Count produces 'count(column)'.
+func (b *SelectBuilder) Count(column string) *SelectBuilder {
+	b.fn("count", column)
+	return b
+}
+
+// CountAll produces 'count(*)'.
+func (b *SelectBuilder) CountAll() *SelectBuilder {
+	b.Count("*")
+	return b
+}
+
+// Min produces 'min(column)' aggregation function.
+func (b *SelectBuilder) Min(column string) *SelectBuilder {
+	b.fn("min", column)
+	return b
+}
+
+// Max produces 'max(column)' aggregation function.
+func (b *SelectBuilder) Max(column string) *SelectBuilder {
+	b.fn("max", column)
+	return b
+}
+
+// Avg produces 'avg(column)' aggregation function.
+func (b *SelectBuilder) Avg(column string) *SelectBuilder {
+	b.fn("avg", column)
+	return b
+}
+
+// Sum produces 'sum(column)' aggregation function.
+func (b *SelectBuilder) Sum(column string) *SelectBuilder {
+	b.fn("sum", column)
+	return b
+}
+
+func (b *SelectBuilder) fn(name, column string) *SelectBuilder {
+	b.Columns(name + "(" + column + ")")
+	return b
+}

qb/select_test.go

@@ -78,6 +78,22 @@ func TestSelectBuilder(t *testing.T) {
 			S: "SELECT * FROM cycling.cyclist_name WHERE id=? ALLOW FILTERING ",
 			N: []string{"expr"},
 		},
+		// Add COUNT all
+		{
+			B: Select("cycling.cyclist_name").CountAll().Where(Gt("stars")),
+			S: "SELECT count(*) FROM cycling.cyclist_name WHERE stars>? ",
+			N: []string{"stars"},
+		},
+		// Add COUNT with GROUP BY
+		{
+			B: Select("cycling.cyclist_name").Count("stars").GroupBy("id"),
+			S: "SELECT id,count(stars) FROM cycling.cyclist_name GROUP BY id ",
+		},
+		// Add Min
+		{
+			B: Select("cycling.cyclist_name").Min("stars"),
+			S: "SELECT min(stars) FROM cycling.cyclist_name ",
+		},
 	}
 
 	for _, test := range table {