10/12/2023 0 Comments Mysql join with a subqueryFor example, your inner query can output multiple results. When you join, the requirements for your subquery output aren't as stringent as when you use the WHERE clause. This can be particularly useful when combined with aggregations. The following query produces the same results as the previous example: SELECT *įROM tutorial.sf_crime_incidents_2014_01 incidents It's fairly common to join a subquery that hits the same table as the outer query rather than filtering in the WHERE clause. You may remember that you can filter queries in joins. This is because the subquery is treated as an individual value (or set of values in the IN case) rather than as a table. Note that you should not include an alias when you write a subquery in a conditional statement. However, IN is the only type of conditional logic that will work when the inner query contains multiple results: SELECT * Most conditional logic will work with subqueries containing one-cell results. The above query works because the result of the subquery is only one cell. The following query returns all of the entries from the earliest date in the dataset (theoretically-the poor formatting of the date column actually makes it return the value that sorts first alphabetically): SELECT * You can use subqueries in conditional logic (in conjunction with WHERE, JOIN/ ON, or CASE). Try it out See the answer Subqueries in conditional logic Hint: use tutorial.sf_crime_incidents_cleandate to make your life a little easier. Write a query that displays the average number of monthly incidents for each category. In general, it's easiest to write inner queries first and revise them until the results make sense to you, then to move on to the outer query. If you're having trouble figuring out what's happening, try running the inner query individually to get a sense of what its results look like. What if you wanted to figure out how many incidents get reported on each day of the week? Better yet, what if you wanted to know how many incidents happen, on average, on a Friday in December? In January? There are two steps to this process: counting the number of incidents each day (inner query), then determining the monthly average (outer query): SELECT LEFT(sub.date, 2) AS cleaned_month, Using subqueries to aggregate in multiple stages These next sections provide examples for which subqueries are the best or only way to solve their respective problems. The above examples, as well as the practice problem don't really require subqueries-they solve problems that could also be solved by adding multiple conditions to the WHERE clause. Write a query that selects all Warrant Arrests from the tutorial.sf_crime_incidents_2014_01 dataset, then wrap it in an outer query that only displays unresolved incidents. This isn't practical if you nest many subqueries, so it's fairly common to only indent two spaces or so. The examples in this tutorial are indented quite far-all the way to the parentheses. Most people do this by indenting the subquery in some way. In this case, we've used the name "sub."Ī quick note on formatting: The important thing to remember when using subqueries is to provide some way to for the reader to easily determine which parts of the query will be executed together. Subqueries are required to have names, which are added after parentheses the same way you would add an alias to a normal table. Once the inner query runs, the outer query will run using the results from the inner query as its underlying table: SELECT sub.* It might sound like a no-brainer, but it's important: your inner query must actually run on its own, as the database will treat it as an independent query. If you were to run this on its own, it would produce a result set like any other query. Let's break down what happens when you run the above query:įirst, the database runs the "inner query"-the part between the parentheses: SELECT * Here's an example of a basic subquery: SELECT sub.* Subqueries can be used in several places within a query, but it's easiest to start with the FROM statement. For example, if you wanted to take the sums of several columns, then average all of those values, you'd need to do each aggregation in a distinct step. Subqueries (also known as inner queries or nested queries) are a tool for performing operations in multiple steps. In this lesson, you will continue to work with the same San Francisco Crime data used in a previous lesson. Using subqueries to aggregate in multiple stages.Starting here? This lesson is part of a full-length tutorial in using SQL for Data Analysis.
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