LibSQL: Implement a DESCRIBE TABLE statement

This statement (for now) outputs the name and types of the different
attributes in a table. It's not standard SQL but all DBMSs that I know
of implement a sort of statement for such functionality.

Since the output of DESCRIBE TABLE is just a relation, an internal
schema, `master` was created and a table definition for DESCRIBE into
it. The table definition and the master schema are not accessible by the
user.

Userland/Libraries/LibSQL/AST/Describe.cpp

@@ -0,0 +1,42 @@
+/*
+ * Copyright (c) 2021, Mahmoud Mandour <ma.mandourr@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibSQL/AST/AST.h>
+#include <LibSQL/Database.h>
+#include <LibSQL/Meta.h>
+#include <LibSQL/Row.h>
+
+namespace SQL::AST {
+
+RefPtr<SQLResult> DescribeTable::execute(ExecutionContext& context) const
+{
+    auto schema_name = m_qualified_table_name->schema_name();
+    auto table_name = m_qualified_table_name->table_name();
+
+    auto table_def_or_error = context.database->get_table(schema_name, table_name);
+    auto table_def = table_def_or_error.release_value();
+    if (!table_def) {
+        if (schema_name.is_null() || schema_name.is_empty())
+            schema_name = "default";
+        return SQLResult::construct(SQLCommand::Describe, SQLErrorCode::TableDoesNotExist, String::formatted("{}.{}", schema_name, table_name));
+    }
+
+    auto describe_table_def = context.database->get_table("master", "internal_describe_table").value();
+    NonnullRefPtr<TupleDescriptor> descriptor = describe_table_def->to_tuple_descriptor();
+
+    context.result = SQLResult::construct();
+
+    for (auto& column : table_def->columns()) {
+        Tuple tuple(descriptor);
+        tuple[0] = column.name();
+        tuple[1] = SQLType_name(column.type());
+        context.result->insert(tuple, Tuple {});
+    }
+
+    return context.result;
+}
+
+}