SELECT * 
FROM your_table
WHERE 
    CASE 
        WHEN EXISTS (SELECT 1 FROM users WHERE user_id = ?) 
        THEN some_column = some_value -- 限制条件
        ELSE 1=1 -- 总是为真，相当于无限制
    END;

SELECT * 
FROM your_table
WHERE 
    IF(
        (SELECT COUNT(*) FROM users WHERE user_id = ?) > 0,
        some_column = some_value, -- 限制条件
        1=1 -- 无限制
    );

CREATE PROCEDURE conditional_select(IN user_id_param INT)
BEGIN
    DECLARE user_exists INT;
    
    SELECT COUNT(*) INTO user_exists FROM users WHERE user_id = user_id_param;
    
    IF user_exists > 0 THEN
        SELECT * FROM your_table WHERE some_column = some_value;
    ELSE
        SELECT * FROM your_table;
    END IF;
END;

-- 创建示例表
CREATE TABLE users (
    user_id INT PRIMARY KEY,
    username VARCHAR(50)
);

CREATE TABLE products (
    product_id INT PRIMARY KEY,
    product_name VARCHAR(100),
    category VARCHAR(50),
    price DECIMAL(10,2)
);

-- 解决方案
SELECT * 
FROM products
WHERE 
    CASE 
        WHEN EXISTS (SELECT 1 FROM users WHERE user_id = 123) 
        THEN category = 'Electronics' -- 限制条件
        ELSE 1=1 -- 无限制
    END;