fix #61, introduce the new expression api for generating queries

aggregate.go

@@ -0,0 +1,49 @@
+package qb
+
+import (
+	"fmt"
+)
+
+// Avg function generates "avg(%s)" statement for column
+func Avg(column ColumnElem) AggregateClause {
+	return Aggregate("AVG", column)
+}
+
+// Count function generates "count(%s)" statement for column
+func Count(column ColumnElem) AggregateClause {
+	return Aggregate("COUNT", column)
+}
+
+// Sum function generates "sum(%s)" statement for column
+func Sum(column ColumnElem) AggregateClause {
+	return Aggregate("SUM", column)
+}
+
+// Min function generates "min(%s)" statement for column
+func Min(column ColumnElem) AggregateClause {
+	return Aggregate("MIN", column)
+}
+
+// Max function generates "max(%s)" statement for column
+func Max(column ColumnElem) AggregateClause {
+	return Aggregate("MAX", column)
+}
+
+// Aggregate generates a new aggregate clause given function & column
+func Aggregate(fn string, column ColumnElem) AggregateClause {
+	return AggregateClause{fn, column}
+}
+
+// AggregateClause is the base struct for building aggregate functions
+type AggregateClause struct {
+	fn string
+	column ColumnElem
+}
+
+// Build compiles the aggregate clause and returns the sql and bindings
+func (c AggregateClause) Build(dialect Dialect) (string, []interface{}) {
+	bindings := []interface{}{}
+	sql := fmt.Sprintf("%s(%s)", c.fn, dialect.Escape(c.column.Name))
+	return sql, bindings
+}
+

aggregate_test.go

@@ -0,0 +1,15 @@
+package qb
+
+import (
+	"testing"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestAggregates(t *testing.T) {
+	col := Column("id", Varchar().Size(36))
+	assert.Equal(t, Avg(col), Aggregate("AVG", col))
+	assert.Equal(t, Count(col), Aggregate("COUNT", col))
+	assert.Equal(t, Sum(col), Aggregate("SUM", col))
+	assert.Equal(t, Min(col), Aggregate("MIN", col))
+	assert.Equal(t, Max(col), Aggregate("MAX", col))
+}