Add query package with first set of conditions and tests

database/query/condition-exists.go

@@ -0,0 +1,28 @@
+package query
+
+import (
+	"errors"
+)
+
+type existsCondition struct {
+	key      string
+	operator uint8
+}
+
+func newExistsCondition(key string, operator uint8) *existsCondition {
+	return &existsCondition{
+		key:      key,
+		operator: operator,
+	}
+}
+
+func (c *existsCondition) complies(f Fetcher) bool {
+	return f.Exists(c.key)
+}
+
+func (c *existsCondition) check() error {
+	if c.operator == errorPresent {
+		return errors.New(c.key)
+	}
+	return nil
+}