Student Seeking Advice for a CSC Exam - sql

Question

I'm a student taking a course on SQL and DB. My question is this: how does one get good at hand writing queries? Our final exam will consist of many of these questions, and I want to do well. We aren't allowed any sort of reference sheet either, just fyi.
I suppose what I'm asking is: how would you approach this?

Answer 1

In short, You require practice aka hands on sql.

Answer 2

You will probably get many opinions on this from others. Aside from practice and reading, try to ensure you understand the absolute basics and sequence of query.
Always use table.column or alias.column to help prevent any ambiguity of where something is coming from.
Know the overall basic segments of writing a query such as
select
[all your alias.columns comma separated]
from
[your primary and/or JOIN/LEFT JOIN/etc tables]
where
[what is the criteria you are looking for]
AND [use proper parenthesis to prevent ambiguity if so needed]
group by
[any columns if doing aggregates such as count, min, max, avg, etc]
[you need to list all NON-AGGREGATE alias.columns]
having
[if any, such as count(*) > someValue]
order by
[any specific columns and ascending or descending order]
[such as orderDate DESC to put most recent order at top]
In my opinion, getting your FROM clause is one of the most important and I try to always list my table JOIN clauses on first table/alias = second table/alias. Indentation helps here so you can see how you get from one table to the next. At this point, do not think of your filtering (YET), just HOW the tables are related. Then you can add "AND" criteria for something you are specifically looking for from that source.
An example of orders. Looking for customers who ordered in the last 30 days. Start with that source as your first FROM table, everything else off of that. So I start with the orders because I care about WHEN something was ordered. I can then join to customers to get their name.
select
c.LastName,
c.FirstName,
o.OrderDate
from
Orders o
JOIN Customers c
on o.CustomerID = c.CustomerID
where
o.OrderDate > [sql-specific current date - 30 days]
order by
c.LastName,
c.FirstName
Another example of orders that ordered a specific item in the last 30 days. In this case, I could reverse the order of details as specific things being ordered might be smaller granularity vs everything. So, altering above such as
select
c.LastName,
c.FirstName,
o.OrderDate
from
Items i
JOIN OrderDetails od
on i.ItemID = od.ItemID
JOIN Orders o
on od.OrderID = o.OrderID
AND o.OrderDate > [sql-specific current date - 30 days]
JOIN Customers c
on o.CustomerID = c.CustomerID
where
i.ItemDescription = 'SomeThing'
order by
c.LastName,
c.FirstName
Notice my indentation nesting. Personal style preference, but at least you can see how alias i to od, od to o, o to c. In my preference, easier to see the trail of tables and how each are directly related. I also added the "AND" clause to filter out orders within the last 30 days directly in the JOIN to the orders table.
LEFT JOINs, I do the same and keep the criteria directly at the JOIN level. If you put a criteria of a left-join into the WHERE clause (without explicitly handling NULL OR [condition] it turns a left-join into an [INNER] join.
Hope this basic guidance helps you get more comfortable as you get more into writing your own queries and course/test preparation.