UNION子句

UNION将多条查询语句连接起来进行展示, 按列进行连接

-- UNION 合并返回结果
SELECT 
	order_id,
    order_date,
    'Active' AS status
FROM orders
WHERE order_date >= '2019-01-01'
UNION
SELECT 
	order_id,
    order_date,
    'Actived' AS status
FROM orders
WHERE order_date < '2019-01-01';

-- 多行合并不支持
SELECT first_name, last_name
FROM customers
UNION
SELECT name
FROM shippers;

-- UNION 顺序, 这个多条件后面可以用CASE WHEN END条件语句
SELECT 
	customer_id,
    first_name,
    points,
    'BRonze' AS type
FROM customers
WHERE points < 2000
UNION
SELECT 
	customer_id,
    first_name,
    points,
    'Silver' AS type
FROM customers
WHERE points between 2000 and 3000
UNION
SELECT 
	customer_id,
    first_name,
    points,
    'Gold' AS type
FROM customers
WHERE points > 3000
ORDER BY first_name;