24May 2021 by Grant Fritchey 2 Comments

Database Fundamentals #30: Create Foreign Keys With T-SQL

Database Fundamentals
You can create foreign keys using TSQL roughly the same way as you created primary keys. You can either use the ALTER TABLE statement to add the foreign key, or, if you already have the parent table created along with it’s primary key, you can use the CREATE TABLE statement to include foreign key constraints. The restrictions for creating foreign keys are still the same when using TSQL. Adding a Foreign Key Using the ALTER TABLE statement is very straight forward as before. This script will create a foreign key relationship between the Personnel.Person table and the Personnel.PersonAddress table: ALTER TABLE Personnel.PersonAddress ADD CONSTRAINT PersonAddress_FK_Person FOREIGN KEY (PersonID) REFERENCES Personnel.Person (PersonID); The ALTER TABLE and ADD CONSTRAINT statements are the same as what you saw before. They respectively refer to…
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14Oct 2019 by Grant Fritchey 1 Comment

Database Fundamentals #23: Filtering Data

Database Fundamentals, SQL Server
If you've been reading these Database Fundamentals posts, you've already seen the WHERE clause because of your use of it when manipulating data with DELETE and UPDATE statements. It's also been used several times earlier in this series to limit the values returned from a SELECT statement. The primary places where people run into trouble with T-SQL is in the JOIN criteria and the WHERE clause criteria. This occurs because they don’t understand well enough what the filters and operators they’re using will do. They end up returning too much data because they didn’t us the WHERE clause or misapplied it. They also filter too much data out. Just remember, there are even more functions than we go over here in this series. While these basic operators answer most needs,…
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23Sep 2019 by Grant Fritchey 8 Comments

Database Fundamentals #22: Using the Join Operator, CROSS JOIN

Database Fundamentals
While the CROSS JOIN is not used much, and, depending on the size of your data it can be dangerous, there are some uses for it. For example, you may want to write a query that will summarize all the sales for the companies and people in your system. You can do this using what is called an aggregate operation or a GROUP BY: SELECT c.CompanyName, p.LastName, SUM(ft.TransactionAmount) AS 'TransactionTotals' FROM Management.Company AS c JOIN Finance.FinancialTransaction AS ft ON c.CompanyID = ft.CompanyID JOIN Personnel.Person AS p ON p.PersonID = ft.PersonID GROUP BY c.CompanyName, p.LastName; This will add all the values up in the SUM operation for each company and each person that has values so that your data will look like this: The only problem with this is, you can’t…
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15Apr 2019 by Grant Fritchey 2 Comments

Database Fundamentals #21: Using the JOIN Operator, OUTER JOIN

Database Fundamentals, SQL Server
The OUTER JOIN returns one complete set of data and then the matching values from the other set. The syntax is basically the same as INNER JOIN but you have to include whether or not you’re dealing with a RIGHT or a LEFT JOIN. The OUTER word, just like the INNER key word, is not required. OUTER JOIN Imagine a situation where you have a list of people. Some of those people have financial transactions, but some do not. If you want a query that lists all people in the system, including those with financial transactions, the query might look like this: SELECT p.LastName, ft.TransactionAmount, ft.TransactionDate, ft.TransactionTime FROM Personnel.Person AS p LEFT JOIN Finance.FinancialTransaction AS ft ON p.PersonID = ft.PersonID; Except for the addition of the LEFT key word, this…
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03Dec 2018 by Grant Fritchey 9 Comments

Database Fundamentals #20: Using the JOIN Operator, Inner Join

Database Fundamentals
It is entirely possible to try to JOIN two tables on almost any field, as long as the two data types can, in some way, be made to reconcile to each other, you can try to join the tables. But, most database designs assume a much more directly relationship and provide a column or columns in one table that match the identifying column or columns in the other table. INNER JOIN The INNER JOIN will return the parts of both data sets that match. Frequently, what you'll see when joining two tables is the same column name in each table. With that, you have to be sure to identify the owner of each column. I've introduced what is called an alias to make it so I don't have to type…
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