SQL is an acronym for Structured Query Language so when getting started with it I think it’s best to think about how it was put together to be able to ask questions about data. With that in mind let’s a take a look at some beginning SQL to understand how to write queries.
Queries can be very simple but they won’t answer very many questions in their most simple forms:
Will not a table and filter are clearly not required to write a query, the most common use for writing a query will require 3 parts to be useful:
- a SELECT-ion of things (columns)
- FROM a a collection of things (a table)
- WHERE a criteria is met (filter(s))
I happen to have a database of movie data. Let’s explore some common filter operators by asking some questions about movies. Filters based on mathematical operators are the easiest to start with. We can ask for films with exactly 2 Oscar nominations or more than 4 or less than 6. We make these filters exclusive or inclusive by adding the “=” operator to the comparison. We can also exclude a specific number by using “<>” (Microsoft SQL Server will also support != but this is outside the ANSI standard so “<>” would be best practice as it will be usable in any version of SQL.)
We can have multiple conditions in our WHERE clause and these are combined with AND or OR statements. We can ask for films with 1, 2 or 3 Oscar nominations (but we’ll also see we have a new operator IN that is cleaner and accomplishes the same goal). We can check for movies released on or after Jan 1, 2017 and were nominated for at least 1 Oscar. We can ask about movies released before the year 1950 that made more money than they spent.
If you’re just getting started with writing SQL queries I hope this was helpful! Next up I’ll cover a few tips for working with dates as well as specific operators for filtering strings.
 It’s important to note that SQL does not guarantee any sort of order in your results and thus the last part of a query is generally an ORDER BY statement that tells SQL what order to put the results in.