You don't have to select all the fields, and you may select them in any order you desire. JOIN Teachers ON TeacherStudent. In such cases, the tablename. Also, when you work with more than one table you must specify the table name as well as the column name for each field where the field name appears in more than one table. In other words, if the column name is not unique among all fields in all tables in the FROM clause, the server will raise an error, because it can't distinguish the table from which to extract the data.
Outer joins return all the values from one of the tables even if there's no matching key.
A calculated value is a result of an operation on one or more columns in multiple rows. For example, a sum, average, or total. In T-SQL, calculated values are called aggregates , and the functions are aggregate functions because they aggregate, or collect a number of values into a single value using a calculation. You've seen the rudiments of how to select data. Selecting data doesn't change it, so selecting is a safe operation.