在筛选查询中，exists通常用于判断某个字段是否存在，常见的应用包括：
SELECT * FROM table_name
WHERE EXISTS (SELECT 1 FROM another_table WHERE condition);
SELECT column_name
FROM table_name
WHERE EXISTS (SELECT column_name FROM another_table WHERE condition);
SELECT * FROM table_name
WHERE EXISTS (SELECT column_name FROM another_table WHERE column_name IS NOT NULL);
SELECT column_name
FROM table_name
WHERE EXISTS (SELECT column_name FROM another_table WHERE table_name.id = another_table.id);
