mysql多库查询if条件

SELECT *
FROM db1.users u
LEFT JOIN (
    SELECT *
    FROM db2.orders o
    WHERE o.user_id = u.id
) o ON 1=1;

SELECT 'user' AS type, id, name
FROM db1.users
UNION ALL
SELECT 'order' AS type, order_id, product_name
FROM db2.orders;

-- 创建Federated Table
CREATE TABLE db1.users_fed (
    id INT,
    name VARCHAR(255)
) ENGINE=FEDERATED CONNECTION='mysql://username:password@hostname:port/db1/users';

-- 创建中间表
CREATE TABLE intermediate_table (
    id INT,
    name VARCHAR(255),
    order_id INT,
    product_name VARCHAR(255)
);

-- 插入数据
INSERT INTO intermediate_table (id, name, order_id, product_name)
SELECT u.id, u.name, o.order_id, o.product_name
FROM db1.users u
LEFT JOIN db2.orders o ON u.id = o.user_id;