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/AST.h

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2021, Tim Flynn <trflynn89@serenityos.org>
+ * Copyright (c) 2021, Mahmoud Mandour <ma.mandourr@gmail.com>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -1050,4 +1051,18 @@ private:
     RefPtr<LimitClause> m_limit_clause;
 };
 
+class DescribeTable : public Statement {
+public:
+    DescribeTable(NonnullRefPtr<QualifiedTableName> qualified_table_name)
+        : m_qualified_table_name(move(qualified_table_name))
+    {
+    }
+
+    NonnullRefPtr<QualifiedTableName> qualified_table_name() const { return m_qualified_table_name; }
+    RefPtr<SQLResult> execute(ExecutionContext&) const override;
+
+private:
+    NonnullRefPtr<QualifiedTableName> m_qualified_table_name;
+};
+
 }

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;
+}
+
+}

Userland/Libraries/LibSQL/AST/Parser.cpp

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2021, Tim Flynn <trflynn89@serenityos.org>
+ * Copyright (c) 2021, Mahmoud Mandour <ma.mandourr@gmail.com>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -46,6 +47,8 @@ NonnullRefPtr<Statement> Parser::parse_statement()
         return parse_alter_table_statement();
     case TokenType::Drop:
         return parse_drop_table_statement();
+    case TokenType::Describe:
+        return parse_describe_table_statement();
     case TokenType::Insert:
         return parse_insert_statement({});
     case TokenType::Update:
@@ -181,6 +184,16 @@ NonnullRefPtr<DropTable> Parser::parse_drop_table_statement()
     return create_ast_node<DropTable>(move(schema_name), move(table_name), is_error_if_table_does_not_exist);
 }
 
+NonnullRefPtr<DescribeTable> Parser::parse_describe_table_statement()
+{
+    consume(TokenType::Describe);
+    consume(TokenType::Table);
+
+    auto table_name = parse_qualified_table_name();
+
+    return create_ast_node<DescribeTable>(move(table_name));
+}
+
 NonnullRefPtr<Insert> Parser::parse_insert_statement(RefPtr<CommonTableExpressionList> common_table_expression_list)
 {
     // https://sqlite.org/lang_insert.html

Userland/Libraries/LibSQL/AST/Parser.h

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2021, Tim Flynn <trflynn89@serenityos.org>
+ * Copyright (c) 2021, Mahmoud Mandour <ma.mandourr@gmail.com>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -59,6 +60,7 @@ private:
     NonnullRefPtr<CreateTable> parse_create_table_statement();
     NonnullRefPtr<AlterTable> parse_alter_table_statement();
     NonnullRefPtr<DropTable> parse_drop_table_statement();
+    NonnullRefPtr<DescribeTable> parse_describe_table_statement();
     NonnullRefPtr<Insert> parse_insert_statement(RefPtr<CommonTableExpressionList>);
     NonnullRefPtr<Update> parse_update_statement(RefPtr<CommonTableExpressionList>);
     NonnullRefPtr<Delete> parse_delete_statement(RefPtr<CommonTableExpressionList>);

Userland/Libraries/LibSQL/AST/Token.h

@@ -1,6 +1,7 @@
 /*
  * Copyright (c) 2021, Tim Flynn <trflynn89@serenityos.org>
  * Copyright (c) 2021, Jan de Visser <jan@de-visser.net>
+ * Copyright (c) 2021, Mahmoud Mandour <ma.mandourr@gmail.com>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -53,6 +54,7 @@ namespace SQL::AST {
     __ENUMERATE_SQL_TOKEN("DEFERRED", Deferred, Keyword)                  \
     __ENUMERATE_SQL_TOKEN("DELETE", Delete, Keyword)                      \
     __ENUMERATE_SQL_TOKEN("DESC", Desc, Keyword)                          \
+    __ENUMERATE_SQL_TOKEN("DESCRIBE", Describe, Keyword)                  \
     __ENUMERATE_SQL_TOKEN("DETACH", Detach, Keyword)                      \
     __ENUMERATE_SQL_TOKEN("DISTINCT", Distinct, Keyword)                  \
     __ENUMERATE_SQL_TOKEN("DO", Do, Keyword)                              \