Beginning SQL

❑ Use IN with your subqueries. When you write a query similar to the following, the database
system has to get all the results from the subquery to make sure that it returns only one value:
SELECT FirstName, LastName
FROM MemberDetails
WHERE MemberId = (SELECT MemberId FROM Orders WHERE OrderId = 2);
If you rewrite the query using the IN operator, the database system only needs to get results
until there’s a match with the values returned by the subquery; it doesn’t necessarily have to get
all the values:
SELECT FirstName, LastName
FROM MemberDetails
WHERE MemberId IN (SELECT MemberId FROM Orders WHERE OrderId = 2);
❑ Avoid using SELECT * FROM. Specifying which columns you need has a few advantages, not all
of them about efficiency. First, it makes clear which columns you’re actually using. If you use
SELECT * and actually use only two out of seven of the columns, it’s hard to guess from the SQL
alone which ones you’re using. If you say SELECT FirstName, LastName...then it’s quite
obvious which columns you’re using. From an efficiency standpoint, specifying columns reduces
the amount of data that has to pass between the database and the application connecting to the
database. This is especially important where the database is connected over a network.
❑ Search on integer columns. If you have a choice, and often you don’t, search on integer columns.
For example, if you are looking for the member whose name is William Doors and whose
MemberId is 13, then it makes sense to search via the MemberId because it’s much faster. In the
Film Club database, it’s also the primary key column, so there’s even more of a speed boost.
Summary
This chapter looked at how to tackle difficult query writing. You examined some fairly involved queries
and learned how to break them down into manageable chunks, and then you saw a few tips on how to
increase the efficiency of your queries. Specifically, this chapter covered the following topics:
❑ Working out what the question is and what it actually involves
❑ How to create complex queries by following a basic process:
❑ Creating the SELECT list first
❑ Looking at the FROM clause and how you need to obtain your data
❑ Tackling difficult queries
❑ Using tips to write more efficient queries
Advanced Queries
By now, you should be familiar enough with SQL to design databases; populate tables and columns in
those databases; add, insert, retrieve, and delete data from your databases; and write complex queries
that obtain exactly the information you want. The chapters that follow cover topics more tangentially
related to SQL, such as views, transactions, security, and methods for tuning your database so that it performs
optimally. The next chapter covers views and discusses how to use them to pull specific data for
specific users and create coherent data sets that are of interest to a particular user or application.
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