https://www.w3school.com.cn/sql/sql_datatypes.asp

暂停执行：--（注释）

加一行新列并命名：

SELECT (points+10) * 100 AS 'discount factor'

删除重复项：

SELECT DISTINCT

不等于：!= 或者 <>
等于条件一个等号 WHERE state = 'va'
日期也要加上单引号 WHERE birthdate > '1990-01-01'

多个条件：
WHERE birthdate > '1990-01-01' AND points > 1000
WHERE birthdate > '1990-01-01' OR points > 1000
AND运算符优先于OR运算符

IN运算符

WHERE state IN ('VA' , 'CA', 'WS')
WHERE quantity IN (49, 39, 23)

BETWEEN运算符

WHERE points BETWEEN 100 AND 300
（两边是等于）

LIKE运算符

WHERE last_name LIKE 'b%'
（查询以B为开头的）
WHERE last_name LIKE 'b____y'
WHERE number NOT LIKE '%9'
（查询尾号不是9的）

REGEXP

WHERE last_name REGEXP '^filed'
（以field为开头）
WHERE last_name REGEXP 'filed$'
（以field为结尾）
WHERE last_name REGEXP 'filed|mac'
（含有field或mac，三个也可以）
WHERE last_name REGEXP '[gim]e'
（含有ge或ie或me都可以）
WHERE last_name REGEXP '[a-h]e'
（含a-h任意字母与e）

IS NULL

WHERE phone IS NULL
WHERE phone IS NOT NULL

ORDER BY

如果没有，就按照主列排序（如id）
ORDER BY first_name DESC
DESC表示降序排列
ORDER BY state, first_name
（按洲排序，相同的洲按名字排序）

LIMIT

LIMIT 3 只显示前三行
LIMIT 6, 3（6为偏移量，跳过6个再选后面三个）

连接

INNER JOIN内连接

INNER可以不写
FROM orders
JOIN customers
ON orders.customer_id = customer.customer_id
（将customer文件中customer_id与order文件中相等的那一部分，加入order文件中）
【根据ID将两个表合并】

定义简写：
FROM order_items oi
JOIN products p ON
（如果给了表一个别名，所有其他地方也一定要用这个别名）

跨数据库连接

SELECT *
FROM order_items oi
JOIN sql_inventory.product p
ON oi.product_id = p.product_id

自连接

SELECT *
FROM employees e
JOIN employees m
ON e.reports_to = m.emplyee_id
（用两个不同的简写名称给同一个表）

SELECT
e.employee_id,
e.first_name,
m.first_name AS manager
FROM employees e
JOIN employees m

隐式连接语法

（以下两种方式结果相同，下面一种为隐式连接语法）
SELECT *
FROM orders o
JOIN customer c
ON o.customer_id = c.customer_id

----Implpicit Join Syntax
SELECT *
FROM order o, customer c
WHERE o.customer_id = c.customer_id
（如果没有WHERE那一行就会交叉连接）

左连接

SELECT
c.customer_id,
c.first_name,
o.order_id
FROM customers c
LEFT JOIN orders o
ON c.customer_id = o.customer_id
ORDER BY c.customer_id
（xianshi左表即customers里的全部内容，不管条件正确还是错误）

右连接：右表全部返回

多表外连接

SELECT
c.customer_id,
c.first_name,
o.order_id
sh.name AS shipper
FROM custimers c
LEFT JOIN orders o
ON c.customer_id = o.customer_id
LEFT JOIN shippers sh
ON o.shipper_id = sh.shipper_id
ORDER BY c.customer_id

USING子句（替换ON语句）

SELECT
o.order_id,
c.first_name
FROM orders o
JOIN customers c
USING (customer_id)
JOIN shippers sh
USING (shipper_id)

USING关键词只能在不同表中额列名字完全一样的情况下使用

自然连接

基于共同列连接
SELECT
order_id,
first name
FROM order o
NATURAL JOIN customers c

交叉连接

隐式语法
SELECT
sh.name AS shipper,
p.name AS product
FROM shippers sh, product
ORDER BY sh.name

显式语法
SELECT
sh.name AS shipper,
p.name AS product
FROM shippers sh
CROSS JOIN products p
ORDER BY sh.name

联合

SELECT
order_id,
order_data,
'Active' AS status #改名
FROM orders
WHERE order_date >= '2019-01-01'
UNION #合并上下查询的记录
SELECT
order_id,
order_data,
'Actived' AS status #改名
FROM orders
WHERE order_date < '2019-01-01'

---

SELECT first_name
FROM customers
UNION
SELECT name
FROM shipper
（显示一列名字，是customers表和shippers表的联合）
（查询返回的列的数量一定要相等）

如果要把联合之后的列名修改
SELECT first_name AS full name

练习：根据不同积分算等级（青铜白银黄金）
SELECT customer_id, first_name, points, 'Bronze'
FROM customers
WHERE points < 2000
UNION
SELECT customer_id, first_name, points, 'Sliver'
FROM customers
WHERE points BETWEEN 2000 AND 3000
UNION
……
ORDER BY first_name