Для столбцов конкретной таблицы
select COLUMN_NAME, DATA_TYPE, character_maximum_length, is_nullable from INFORMATION_SCHEMA.COLUMNS where TABLE_NAME = 'table_name'
Для всех столбцов таблиц
select COLUMN_NAME, DATA_TYPE, character_maximum_length, is_nullable from INFORMATION_SCHEMA.COLUMNS
Для ограничений
select * from INFORMATION_SCHEMA.CONSTRAINT_COLUMN_USAGE where table_name ='Table_name'