sql复习

1. 创建数据库
   CREATE DATABASE helloworld DEFAULT CHARACTER SET utf8 COLLATE utf8_unicode_ci;
   2.create table student(
   PersonID int default 0,
   LastName varchar(255) not null
   )
   3.select
   3.1 distinct去重
   SELECT DISTINCT country FROM Websites;
   3.2 top百分比选取
   SELECT TOP 50 PERCENT * FROM Websites;

   3.3 into复制原表到新表
   SELECT *INTO newtable [IN externaldb] FROM table1;
   3.3 INSERT INTO SELECT
   INSERT INTO table2 SELECT * FROM table1;

2. limit
   SELECT * FROM Persons LIMIT 5;

3. in WHERE 子句中规定多个值。
   SELECT * FROM Websites WHERE name IN ('Google','菜鸟教程');
   6.like
   SELECT * FROM Websites WHERE name LIKE 'G%';查找G开头
   SELECT * FROM Websites WHERE name LIKE '%oo%';
   7.inner join 两个表合并记录 on 条件
   SELECT Websites.id, Websites.name, access_log.count, access_log.date
   FROM Websites
   INNER JOIN access_log
   ON Websites.id=access_log.site_id;

4. 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

5. IGHT JOIN 关键字从右表（table2）返回所有的行，即使左表（table1）中没有匹配。如果左表中没有匹配，则结果为 NULL。

6. union 合并结果 SELECT column_name(s) FROM table1 UNION SELECT column_name(s) FROM table2;