added alter table command to fluent query builder

source/orm/query/internal/key_value_pair.cpp

@@ -15,15 +15,19 @@ key_value_pair::key_value_pair(const sql::column &col, utils::database_type valu
 }
 
 key_value_pair::key_value_pair(const char *name, utils::database_type value)
-  : name_(name)
-  , value_(std::move(value)) {
+: name_(name)
+, value_(std::move(value)) {
 }
 
+key_value_pair::key_value_pair( const char* name, utils::placeholder p )
+: name_(name)
+, value_(p) {}
+
 const std::string &key_value_pair::name() const {
   return name_;
 }
 
-const utils::database_type& key_value_pair::value() const {
+const std::variant<utils::placeholder, utils::database_type>& key_value_pair::value() const {
   return value_;
 }
 }

source/orm/query/internal/query_parts.cpp

@@ -46,6 +46,13 @@ const std::string& query_drop_key_constraint_part::name() const {
   return name_;
 }
 
+query_drop_foreign_key_constraint_part::query_drop_foreign_key_constraint_part()
+: query_part( sql::dialect_token::DropConstraint ) {}
+
+void query_drop_foreign_key_constraint_part::accept( query_part_visitor& visitor ) {
+    visitor.visit(*this);
+}
+
 query_add_foreign_key_constraint_part::query_add_foreign_key_constraint_part(const std::vector<sql::column>& columns)
 : query_part(sql::dialect_token::ForeignKey)
 , columns_(columns) {}
@@ -357,6 +364,18 @@ void query_create_table_part::accept(query_part_visitor &visitor)
   visitor.visit(*this);
 }
 
+query_create_schema_part::query_create_schema_part(std::string schema)
+: query_part( sql::dialect_token::Schema )
+, schema_( std::move( schema ) ){}
+
+const std::string& query_create_schema_part::schema() const {
+    return schema_;
+}
+
+void query_create_schema_part::accept( query_part_visitor& visitor ) {
+    visitor.visit(*this);
+}
+
 query_drop_part::query_drop_part()
 : query_part(sql::dialect_token::Drop) {}
 
@@ -379,4 +398,15 @@ void query_drop_table_part::accept(query_part_visitor &visitor)
   visitor.visit(*this);
 }
 
+query_drop_schema_part::query_drop_schema_part(std::string schema_)
+: query_part( sql::dialect_token::Schema )
+, schema_( std::move( schema_ ) ) {}
+
+const std::string& query_drop_schema_part::schema() const {
+  return schema_;
+}
+
+void query_drop_schema_part::accept( query_part_visitor& visitor ) {
+  visitor.visit(*this);
+}
 }