Difference between Alter and Update SQL

ALTER is a DDL (Data Definition Language) statement. Whereas UPDATE is a DML (Data Manipulation Language) statement. ALTER is used to update the structure of the table (add/remove field/index etc). Whereas UPDATE is used to update data.

ALTER is used to change things like table structures or stored procs, otherwise known as DDL statements.

ALTER table MyTable 
ADD MyNewColumn VARCHAR(100)

OR

ALTER PROC dbo.MyStoredProc

The ALTER changes the table in the database, you can add or remove columns, etc. But it does not change data (except in the dropped or added columns of course).

While the UPDATE changes the rows in the table, and leaves the table unchanged.