數據庫字符串操作

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'
當搜索條件中有單引號時，兩個單引號表示一個單引號