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SQL Mastery: Commands for Relational Databases

Learn advanced SQL commands for relational databases, including DISTINCT, GROUP BY, ORDER BY, and more. Practice managing data effectively and efficiently. Get ready for a test review after the lab session.

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SQL Mastery: Commands for Relational Databases

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  1. CS 111 – Nov. 10 • Structured Query Language (SQL) • We’ve already seen simple select statements, with optional “where” clause and aggregate functions. • More example commands today • Relational database • Commitment • Review for test after lab

  2. Log in • As you will see in lab, you first need to “log in” to the database management system in order to see your database. • From a Web browser, go to cs.furman.edu/phpMyAdmin/ • Enter your database username & password • (Demo) • If your SQL commands are incorrect, you’ll get error message.

  3. Example An abridged Employee table:

  4. Distinct • Sometimes in SQL you have a lot of repeated data. And you only want the values themselves, not the repetitions. • Ex. 70, 70, 70, 80, 80, 80, 80, 80, 100  70, 80, 100 • Examples select Test2 from Student; gives all test grades select distinct Test2 from Student; only shows each value once select distinct Test1, Test2 from Student; finds distinct pairs. 70/60, 70/80, and 90/80 are considered distinct.

  5. Group by • The “group by” clause is good at finding subtotals. • Example: how many employees by job title: • Select count(first) from Employee group by Title; • Actually, doesn’t matter which field we count. • To make output easier to understand, we should also print out the job titles: • Select Title, count(Salary), avg(Salary) from Employee group by title; • We can even subtotal by 2 fields: What would this command mean? • Select Location, Title, count(Salary) from Employee group by Location, Title;

  6. Order by • This clause is used for sorting. • Default order is ascending. • select * from Employee order by Last; • select * from Employee order by Location, Last; • select * from Employee where Salary > 50000 order by Location, Title;

  7. More on “where” • Boolean conditions: when you use the “where” clause can include the word “and” or “or” to make complex conditions. • Ex. What if we wanted salaries of employees with names starting with M or P. • Use “in” when you want to select among several possible values. Has the same effect as “or” • Select * from Employee where Location in (“Dublin”, “Chicago”); • Use “between” for an (inclusive) range of values to check • Select * from Employee where Salary between 60000 and 70000;

  8. Relational Database • Databases with just 1 table are not very powerful. • More interesting if 2 tables are related. • Books and publishers • Customers and orders • Pets and owners • Typically we have a “one-to-many” relationship • The two tables need to have a field in common. • You can see this if you try to list the necessary fields in the above examples. • In SQL, to refer to a field within some table, we use the dot notation: Customer.First, Pet.Name, Order.ID

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