数据库问题汇总
1.util.Date类型数据插入mysql时数据库时，日期会少13小时
在连接参数中添加“serverTimezone=Asia/Shanghai”。
2.定义band字段在 Mybatis中关于OGNL表达式会出现冲突：Malformed OGNL expression: band != null
表字段为band，band是捆绑的意思，与mybatis的OGNL表达式发生冲突。可能发生冲突的变量集合
- bor 字符|
- xor 字符^
- and 字符&&
- band 字符&
- eq 字符==
- neq 字符！=
- lt 字符<
- gt 字符>
- lte 字符<=
- gte 字符>=
- shl 字符 <<
- shr 字符>>
- ushr 字符>>>
3.com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: Table 'test.QRTZ_TRIGGERS' doesn't exist
问题：数据库中明这张表 qrtz_triggers 还是报这个错
原因：这个错报的是在数据库test下面没有这个表【QRTZ_TRIGGERS】。其实数据库中有的是【qrtz_triggers】。区别在于一个是大写，一个是小写。默认情况下，mysql是区分大小写的，所以为了避免这种问题，就需要把 mysql 的区分大小写的属性给修改了。
解决办法：
用root登录，修改 /etc/my.cnf。在[mysqld]节点下，加入一行： lower_case_table_names=1
CentOS 7下mysql停止和重启：
/bin/systemctl stop mysqld.service
/bin/systemctl restart mysqld.service
4.各数据库表名和字段名长度限制
5.mysql修改端口号
登录mysql：
mysql -u root -p
查看当前端口：
show global variables like 'port';
修改 my.cnf 配置文件的 port 属性：
port=要修改的端口号
重启mysql：
service mysqld restart
6.UNION ALL为什么会降低性能
High Performance MySQL, Second Edition P195 Optimizing UNION MySQL always executes UNION queries by creating a temporary table and filling it with the UNION results. MySQL can’t apply as many optimizations to UNION queries as you might be used to. You might have to help the optimizer by manually “pushing down” WHERE, LIMIT, ORDER BY, and other conditions (i.e., copying them, as appropri- ate, from the outer query into each SELECT in the UNION). It’s important to always use UNION ALL, unless you need the server to eliminate dupli- cate rows. If you omit the ALL keyword, MySQL adds the distinct option to the tem- porary table, which uses the full row to determine uniqueness. This is quite expensive. Be aware that the ALL keyword doesn’t eliminate the temporary table, though. MySQL always places results into a temporary table and then reads them out again, even when it’s not really necessary (for example, when the results could be returned directly to the client).
union 会生成临时表，整个过程就相当于是先写再读，所以在某些情况下速度会变慢很多。
7.mybatis中大于等于小于等于的写法
大于等于 <![CDATA[ >= ]]> 小于等于 <![CDATA[ <= ]]> 例如：sql如下： create_date_time <![CDATA[ >= ]]> #{startTime} and create_date_time <![CDATA[ <= ]]> #{endTime}