MySQL合并数据和给表，字段取别名

union关键字
SELECT s_id, f_name, f_price 
FROM fruits
WHERE f_price < 9.0
UNION ALL
SELECT s_id, f_name, f_price 
FROM fruits
WHERE s_id IN(101,103);

为表取别名
SELECT * from orders AS o  WHERE o.o_num = 30001;

SELECT c.c_id, o.o_num
FROM customers AS c LEFT OUTER JOIN orders AS o
ON c.c_id = o.c_id;
为字段取别名
SELECT f1.f_name AS fruit_name, f1.f_price AS fruit_price
FROM fruits AS f1
WHERE f1.f_price < 8;

SELECT CONCAT(RTRIM(s_name) , ' (',  RTRIM(s_city), ')')
FROM suppliers
ORDER BY s_name;

SELECT CONCAT(RTRIM(s_name) , ' (',  RTRIM(s_city), ')')
as suppliers_title
FROM suppliers

ORDER BY s_name;

【例.62】查询所有价格小于9的水果的列表，查询s_id等于101和103所有的水果种类，使用UNION连接查询结果
SELECT s_id, f_name, f_price 
FROM fruits
WHERE f_price < 9.0
UNION ALL
SELECT s_id, f_name, f_price 
FROM fruits
WHERE s_id IN(101,103);

【例.63】查询所有价格小于9的水果的列表，查询s_id等于101和103所有的水果种类，使用UNION ALL连接查询结果，SQL语句如下
SELECT s_id, f_name, f_price 
FROM fruits
WHERE f_price < 9.0
UNION ALL
SELECT s_id, f_name, f_price 
FROM fruits
WHERE s_id IN(101,103);

【例.64】为orders表取别名o，查询订30001订单的下单日期
SELECT * from orders AS o 
WHERE o.o_num = 30001;

【例.65】为customers和orders表分别取别名，并进行连接查询
SELECT c.c_id, o.o_num
FROM customers AS c LEFT OUTER JOIN orders AS o
ON c.c_id = o.c_id;

【例.66】查询fruits表，为f_name取别名fruit_name，f_price取别名fruit_price，为fruits表取别名f1，查询表中f_price < 8的水果的名称
SELECT f1.f_name AS fruit_name, f1.f_price AS fruit_price
FROM fruits AS f1
WHERE f1.f_price < 8;

【例.67】查询suppliers表中字段s_name和s_city，使用CONCAT函数连接这个两个字段值，并取列别名为suppliers_title。
如果没有对连接后的值取别名，其显示列名称将会不够直观，输入如下SQL，
SELECT CONCAT(RTRIM(s_name) , ' (',  RTRIM(s_city), ')')
FROM suppliers
ORDER BY s_name;