How to update field to add value to existing value?

Question

How to update field to add value to existing value?
For example I have

Table name: table

id   credit
1      4
2      5
3      3
         


        

Answer 1

Simply use credit = credit + 7 instead of credit = '+7' in UPDATE statement

UPDATE tablename SET credit = credit + 7 WHERE id = 1

See this SQLFiddle