Sql Join Queries Assignment Help

I'm basing most of this on just trying to get the "right" answer, so you may discover there are some performance issues. No point in speeding up an incorrect query.

Understand the table relationships - Most will be one to many. Know the "many" table. Identify the fields required for your joins.

Think about LEFT join scenarios - Select all the employees and their paycheck from last month. What if they didn't get a paycheck last month?

Know the result set: 1) In a spreadsheet, manually enter at least one correct record for your query. 2) Write the query in a simple enough form to identify how many records should be returned. Use both of these to test your query to make sure joining a new table doesn't alter the result.

Break up your query into managable parts - You don't have to write it all at once. Complex queries can sometimes just be a collection of simple queries.

Beware of mixed levels of aggregation: If you have to put monthly, quarterly and year-to-date values in the same result set, you'll need to calculate them separately in queries grouped on different values.

Know when to UNION Sometimes it's easier to break up subgroups into their own select statements. If you have a table mixed with managers and other employees, and on each column you have to do Case statements based on membership in one of these groups, it may be easier to write a Manager query and union to an Employee query. Each one would contain their own logic. Having to include items from different tables in different rows is an obvious use.

Complex/Nested formulas - Try to consistently indent and don't be afraid to use multiple lines. "CASE WHEN CASE WHEN CASE WHEN" will drive you nuts. Take the time to think these through. Save the complex calcs for last. Get the correct records selected first. Then you attack complex formulas knowing you're working with the right values. Seeing the values used in the formulas will help you spot areas where you have to account for NULL values and where to handle the divide by zero error.

Test often as you add new tables to make sure you're still getting the desired result set and knowing which join or clause is the culprit.

Let's start with the basics: what is a clause, anyway? If you're familiar with SQL, you've probably used s extensively in the past. But using them is different from being able to explain what they do — so let's go over a high-level definition to start us off.

Simply put, a combines records from two separate tables. Oftentimes, we come upon situations within SQL in which data on two separate tables is linked, but separated. s help bring that data back together.

Here's an example: let's say we have two tables in a database that tracks sales from an e-commerce company. One table is called , and contains data individual customers, like and . The second table is called , and contains information on individual orders that have been placed — like and .

Each in our database is placed by a , but we don't keep the customer's information within the table. Why not? Because if the same customer placed multiple orders, and we kept track of customer information within the table, we'd be duplicating data unnecessarily. By separating customer information into its own table, we can reduce redundancy and make updates and changes much easier to handle.

So, we include a field called within each record on the table. this ID is linked to a on the table, which contains non-redundant data for each individual customer.

When we want to bring two tables together, we use a statement to combine data as necessary.

Here's how these two tables might look in practice:

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