在篩選查詢中，exists通常用于判斷某個字段是否存在，常見的應用包括：
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);
