复习SQL

参考自：runoob

Basis

Basic Usage

/* Select */
SELECT column_name,column_name
FROM table_name;

SELECT * FROM table_name;

/* Insert */
INSERT INTO table_name
VALUES (value1,value2,value3,...);

INSERT INTO table_name (column1,column2,column3,...)
VALUES (value1,value2,value3,...);

/* Update */
UPDATE table_name
SET column1=value1,column2=value2,...
WHERE some_column=some_value;

/* Delete */
DELETE FROM table_name
WHERE some_column=some_value;

Where

SELECT column_name,column_name
FROM table_name
WHERE column_name operator value;

Distinct

Choose the only one

SELECT DISTINCT column_name,column_name
FROM table_name;

Order by

SELECT column_name,column_name
FROM table_name
ORDER BY column_name,column_name ASC|DESC;

Note that ASC is the default way.

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Advanced

演示数据库说明

Websites表

+----+--------------+---------------------------+-------+---------+
| id | name         | url                       | alexa | country |
+----+--------------+---------------------------+-------+---------+
| 1  | Google       | https://www.google.cm/    | 1     | USA     |
| 2  | 淘宝          | https://www.taobao.com/   | 13    | CN      |
| 3  | 菜鸟教程      | http://www.runoob.com/    | 4689  | CN      |
| 4  | 微博          | http://weibo.com/         | 20    | CN      |
| 5  | Facebook     | https://www.facebook.com/ | 3     | USA     |
| 7  | stackoverflow | http://stackoverflow.com/ |   0 | IND     |
+----+---------------+---------------------------+-------+---------+

Access_log 表

+-----+---------+-------+------------+
| aid | site_id | count | date       |
+-----+---------+-------+------------+
|   1 |       1 |    45 | 2016-05-10 |
|   2 |       3 |   100 | 2016-05-13 |
|   3 |       1 |   230 | 2016-05-14 |
|   4 |       2 |    10 | 2016-05-14 |
|   5 |       5 |   205 | 2016-05-14 |
|   6 |       4 |    13 | 2016-05-15 |
|   7 |       3 |   220 | 2016-05-15 |
|   8 |       5 |   545 | 2016-05-16 |
|   9 |       3 |   201 | 2016-05-17 |
+-----+---------+-------+------------+

apps表

+----+------------+-------------------------+---------+
| id | app_name   | url                     | country |
+----+------------+-------------------------+---------+
|  1 | QQ APP     | http://im.qq.com/       | CN      |
|  2 | 微博 APP | http://weibo.com/       | CN      |
|  3 | 淘宝 APP | https://www.taobao.com/ | CN      |
+----+------------+-------------------------+---------+

通配符

在 SQL 中，通配符与 SQL LIKE 操作符一起使用，SQL 通配符用于搜索表中的数据。

通配符	描述
_	代替一个字符
%	代替0或多个字符
[字符串]	字符串中的任意一个字符
[^字符串] 或 [!字符串]	不在字符串中的任意一个字符

SELECT * FROM Websites
WHERE url LIKE 'https%';

正则表达式

/* 正则表达式匹配 */
SELECT * FROM Websites
WHERE name REGEXP '^[GFs]';

/* 正则表达式匹配 否定形式 */
SELECT * FROM Websites
WHERE name NOT REGEXP '^[GFs]';

IN

IN 操作符允许您在 WHERE 子句中规定多个值。

SELECT * FROM Websites
WHERE name IN ('Google','菜鸟教程');

BETWEEN

BETWEEN 操作符用于选取介于两个值之间的数据范围内的值。

SELECT column_name(s)
FROM table_name
WHERE column_name BETWEEN value1 AND value2;

AS

通过使用 SQL，可以为表名称或列名称指定别名。

SELECT column_name AS alias_name
FROM table_name;

JOIN

INNER JOIN

SQL INNER JOIN

语法如下：

SELECT column_name(s)
FROM table1
INNER JOIN table2
ON table1.column_name=table2.column_name;

/*或者*/
SELECT column_name(s)
FROM table1
JOIN table2
ON table1.column_name=table2.column_name;

/* 实例 */
SELECT Websites.name, access_log.count, access_log.date
FROM Websites
INNER JOIN access_log
ON Websites.id=access_log.site_id
ORDER BY access_log.count;

举例如下：

img

LEFT JOIN

LEFT JOIN 关键字从左表（table1）返回所有的行，即使右表（table2）中没有匹配。如果右表中没有匹配，则结果为 NULL。(RIGHT JOIN 则相反)

SQL LEFT JOIN

SELECT column_name(s)
FROM table1
LEFT JOIN table2
ON table1.column_name=table2.column_name;

img

FULL OUTER JOIN

FULL OUTER JOIN 关键字只要左表（table1）和右表（table2）其中一个表中存在匹配，则返回行。

SQL FULL OUTER JOIN

SELECT column_name(s)
FROM table1
FULL OUTER JOIN table2
ON table1.column_name=table2.column_name;

UNION

UNION 操作符用于合并两个或多个 SELECT 语句的结果集。

Attention： UNION 内部的每个 SELECT 语句必须拥有相同数量的列。列也必须拥有相似的数据类型。同时，每个 SELECT 语句中的列的顺序必须相同。

SELECT column_name(s) FROM table1
UNION
SELECT column_name(s) FROM table2;

/*默认地，UNION 操作符选取不同的值。如果允许重复的值，请使用 UNION ALL。*/
SELECT column_name(s) FROM table1
UNION ALL
SELECT column_name(s) FROM table2;

img img

另外，带where的union all会这样：

img

INSERT INTO SELECT

INSERT INTO SELECT 语句从一个表复制数据，然后把数据插入到一个已存在的表中。目标表中任何已存在的行都不会受影响。

INSERT INTO table2
(column_name(s))
SELECT column_name(s)
FROM table1;

EXISTS

结果集是否为空。比如说，选出有人访问的网站：

SELECT * FROM Websites
WHERE EXISTS (SELECT * FROM access_log
             WHERE access_log.site_id = Websites.id)

还有NOT EXISTS搭配使用。

---

SQL Function

GROUP BY

GROUP BY 语句用于结合聚合函数，根据一个或多个列对结果集进行分组。

SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name;

img

HAVING

在 SQL 中增加 HAVING 子句原因是，WHERE 关键字无法与聚合函数一起使用。

HAVING 子句可以让我们筛选分组后的各组数据。

SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name
HAVING aggregate_function(column_name) operator value;

查找访问量大于200的网站

SELECT Websites.name, Websites.url, SUM(access_log.count) AS nums FROM (access_log
INNER JOIN Websites
ON access_log.site_id=Websites.id)
GROUP BY Websites.name
HAVING SUM(access_log.count) > 200;

聚合(Aggregate)函数

SQL Aggregate 函数计算从列中取得的值，返回一个单一的值。

有用的 Aggregate 函数：

• AVG() - 返回平均值
• COUNT() - 返回行数
• FIRST() - 返回第一个记录的值
• LAST() - 返回最后一个记录的值
• MAX() - 返回最大值
• MIN() - 返回最小值
• SUM() - 返回总和