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The location and size of data files

If you really need every column, explicitly list every column. This isn’t so much a rule, but rather, a means of preventing future system errors and additional SQL performance tuning. For example, if you’re using an INSERT... SELECT... and the source table has changed via the addition of a new column, you might run into issues, even if that column isn’t needed by the destination table, e.g.:

INSERT INTO Employees SELECT * FROM OldEmployees

Msg 213, Level 16, State 1, Line 1
Insert Error: Column name or number of supplied values does not match table definition.

To avoid this kind of error from SQL Server, you should declare each column individually:

INSERT INTO Employees (FirstName, City, Country)
SELECT Name, CityName, CountryName
FROM OldEmployees

Note, however, that there are some situations where the use of SELECT * could be appropriate. For example, with temp tables—which leads us to our next topic.

SQL Tuning: The Wise Use of Temporary Tables (#Temp)

Temporary tables usually increase a query’s complexity. If your code can be written in a simple, straightforward manner, I’d suggest avoiding temp tables.

But if you have a stored procedure with some data manipulation that cannot be handled with a single query, you can use temp tables as intermediaries to help you to generate a final result.

When you have to join a large table and there are conditions on said table, you can increase database performance by transferring your data in a temp table, and then making a join on that. Your temp table will have fewer rows than the original (large) table, so the join will finish faster!

The decision isn’t always straightforward, but this example will give you a sense for situations in which you might want to use temp tables:

Imagine a customer table with millions of records. You have to make a join on a specific region. You can achieve this by using a SELECT INTO statement and then joining with the temp table:

SELECT * INTO #Temp FROM Customer WHERE RegionID = 5
SELECT r.RegionName, t.Name FROM Region r JOIN #Temp t ON t.RegionID = r.RegionID

(Note: some SQL developers also avoid using SELECT INTO to create temp tables, saying that this command locks the tempdb database, disallowing other users from creating temp tables.