Add StringColumn type and expression

Add Projection type
Alias refactoring
More numeric operations

sqlbuilder/string_expression.go

@@ -0,0 +1,25 @@
+package sqlbuilder
+
+type StringExpression interface {
+	Expression
+
+	Eq(expression StringExpression) BoolExpression
+	EqL(value string) BoolExpression
+	NotEq(expression StringExpression) BoolExpression
+}
+
+type stringInterfaceImpl struct {
+	parent StringExpression
+}
+
+func (b *stringInterfaceImpl) Eq(expression StringExpression) BoolExpression {
+	return newBinaryBoolExpression(b.parent, expression, []byte(" = "))
+}
+
+func (b *stringInterfaceImpl) EqL(value string) BoolExpression {
+	return newBinaryBoolExpression(b.parent, Literal(value), []byte(" = "))
+}
+
+func (b *stringInterfaceImpl) NotEq(expression StringExpression) BoolExpression {
+	return newBinaryBoolExpression(b.parent, expression, []byte(" != "))
+}