关于数据库索引的有关问题~求教

关于数据库索引的问题~求教~
数据库索引的优缺点是什么？什么是合理索引。以下语句涉及内容均建立了索引，请问其操作是否效率高？如果不是，请写出效率高的方式？
a select content from record where substring(card_no,1,4)='5378'
b select content from record where amount/30=1000
c select content from record where convert(char(10),data,112)='19990513'

对索引实在不解，看了相关资料还是不知道怎么去分辨效率高低问题，请懂的朋友告知~~

------解决方案--------------------

a select content from record where substring(card_no,1,4)='5378'
select content from record where card_no like '5378%'

b select content from record where amount/30=1000
select content from record where amount=1000*30

c select content from record where convert(char(10),data,112)='19990513'
select content from record where data>='19990513' and data<'19990514'



------解决方案--------------------

探讨

a select content from record where substring(card_no,1,4)='5378'
select content from record where card_no like '5378%'

b select content from record where amount/30=1000
select content from record……