数据库字符串操作

1.截取字符串：
    substr('This is a test', 6, 2)          would return 'is'
    substr('This is a test', 6)              would return 'is a test'
    substr('TechOnTheNet', 1, 4)     would return 'Tech'
    substr('TechOnTheNet', -3, 3)    would return 'Net'
    substr('TechOnTheNet', -6, 3)    would return 'The'
    substr('TechOnTheNet', -8, 2)    would return 'On'
2.字符串相加：'aaa '|| 'bbb '

3.select * from a where b='sdfsf''sdfsf'
当搜索条件中有单引号时，两个单引号表示一个单引号