question about extendibility

i want to write a simple project,i want do it simple,and upload to a web host,and update it little by little

but as i concern about the database,if i find some new requirement to add a column in sql server database,

now,i'm confused about if i do that,whether the old data will be missing or not,

if not ,how can i do that?

thanks in advance!

(sorry for my english....)

 
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