MySQL Wizardry

Some help from SQL itself

The Wizard smiled. "If you like to, you're welcome. However, you could get some help from the database engine itself, provided that you ask nicely." And while he was speaking he typed a very cryptic statement:

mysql> SELECT CONCAT(', SUM(IF(dept = "',dept,'", 1,0)) AS `',dept,'`')
    -> FROM departments;

"You know," he went on, "you can also use SQL to produce SQL code. This is one of the cases. You have in front of you the list of columns that you should include in your query. Now, with some cut-and-paste, we could get the result you want. Here."

mysql> SELECT location
    -> , SUM(IF(dept = "Development", 1,0)) AS `Development` 
    -> , SUM(IF(dept = "Personnel", 1,0)) AS `Personnel`
    -> , SUM(IF(dept = "Research", 1,0)) AS `Research`
    -> , SUM(IF(dept = "Sales", 1,0)) AS `Sales`
    -> , SUM(IF(dept = "Training", 1,0)) AS `Training`
    -> , COUNT(*) AS total
    -> FROM locations INNER JOIN employees USING (loc_code)
    -> INNER JOIN departments USING (dept_code)
    -> GROUP BY location;

The wizard was now unstoppable. He had reached the stage where he simply couldn't help giving away his knowledge.

"Before we go on," he said, taking possession of my keyboard, "let's see how this same method can do more than counting. Let's replace those 1s with a numeric field, and do real summing up."

mysql> SELECT location
    -> , SUM(IF(dept = "Development", salary,0)) AS `Development`
    -> , SUM(IF(dept = "Personnel", salary,0)) AS `Personnel`
    -> , SUM(IF(dept = "Research", salary,0)) AS `Research`
    -> , SUM(IF(dept = "Sales", salary,0)) AS `Sales`
    -> , SUM(IF(dept = "Training", salary,0)) AS `Training`
    -> , SUM(salary) AS total
    -> FROM locations INNER JOIN employees USING (loc_code)
    -> INNER JOIN departments USING (dept_code)
    -> GROUP BY location;

"Don't forget to change also the total field. A simple SUM without IF, and your total is ready."

I was looking at the screen, which was showing what seemed to be the complete solution to my problem, but the wizard was shaking his head. "As a matter of fact," he was saying, there is something that we can improve here. We have two queries, in which we are reading the departments table. So we are reading it twice twice. The whole process could be improved, by querying for department code the first time, and omitting the join with departments the second time."

mysql> SELECT CONCAT(', SUM(IF(dept_code = "',dept_code,'", 1,0)) AS `',dept,'`')
    -> FROM departments;

"Here. Let's get the columns once more. Good. And there it is. This one looks better."

mysql> SELECT location
    -> , SUM(IF(dept_code = "1", 1,0)) AS `Personnel`
    -> , SUM(IF(dept_code = "2", 1,0)) AS `Training`
    -> , SUM(IF(dept_code = "3", 1,0)) AS `Research`
    -> , SUM(IF(dept_code = "4", 1,0)) AS `Sales`
    -> , SUM(IF(dept_code = "5", 1,0)) AS `Development`
    -> , COUNT(*) AS total
    -> FROM locations INNER JOIN employees USING (loc_code)
    -> GROUP BY location;

He changed the previous two SQL statements, executed them, with some cut-and-pasting in the middle, and got exactly the same result. Now he was explaining me why he did it. "The first query is scanning all the departments table, and we know that its results will be used to build the second query containing the employees table, which has already a department code. Therefore, we can skip the join with departments, since the only purpose of that join was to get the names of the departments."

"I see" was the only comment I could offer, Since I was overwhelmed by his continuous insight. I got the idea, and I couldn't help thinking that he must have done that before. All those pieces of information were coming just too fast for me. Luckily, all logs were on, so I knew that I would be able to get all the statements back when he would leave. Which was not the case yet. The wizard was now ready to give me his philosophical view of cross tabulating.

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