In SQL, a query requests data using the select command. For example, this query asks for a listing of authors who live in the state of California:
select au_lname, city, state from authors where state = "CA"
Data modification refers to the addition, deletion, or change of data using the insert, delete, or update commands. For example:
insert into authors (au_lname, au_fname, au_id)
values ("Smith", "Gabriella", "999-03-2346")
Other SQL commands, such as dropping tables or adding users, perform administrative operations. For example:
drop table authors
Each command or SQL statement begins with a keyword, such as insert, that names the basic operation performed. Many SQL commands also have one or more keyword phrases, or clauses, that tailor the command to meet a particular need. When you run a query, Transact-SQL displays the results. If no data meets the criteria specified in the query, the user gets a message to that effect. Data modification statements and administrative statements do not retrieve data, and therefore, do not display results. Transact-SQL provides a message to let the user know whether the data modification or other command has been performed.
