ColdFusion Muse

Leveraging Your SQL - Update Using a Join

Mark Kruger July 15, 2005 12:17 PM SQL tips Comments (4)

Most update queries are pretty straightforward. You already know the primary key or some other criteria for a single table and and the WHERE clause is just "WHERE pk_id = 4" or "WHERE area_code = 312". There are times, however, when it might be useful to update a table based on critria from 2 or more tables. For a purely hypothetical example, let's say I have a shipping amount in an "ordShip" table, a base amount in an "orders" table and I have a tax amount (as a decimal) in a tax table based on the state. Let's also assume I have the state in the "orders" table. I want to update the "grandTotal" amount in the "orders" table. The formula would be:

base Amount + (base Amount * tax rate) + shipping.
How would I go about it?

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Why You should worry about your execution plan

Mark Kruger June 28, 2005 11:15 AM SQL tips, MS SQL Server, Coldfusion & Databases Comments (9)

No I'm not talking about dead man walking. I'm talking about your database execution plan. I want to give fair warning to all of you Microsoft haters out there (and you know who you are) that I'm going to use lingo from Microsoft SQL Server 2000. It's a ubiquitous and full-featured database with good documentation regarding this subject. Much of what is said here applies to other databases as well (no doubt using different lingo). So please, feel free to post comments of how Oracle or MySQL or PostgreSQL or Interbase or your flat file - all have a great way of doing this. But please don't post about how your favorite DB is so great and Microsoft is the spawn of Satan. That's not helpful and it's makes me want to poke out my eye with an ice pick! Whew! Now that that's out of the way.

The database execution plan is the series of steps that a database takes to deliver a particular query or task. These steps are cached on the DB server. When you run a query it looks in the cache for a plan that matches. If it finds one, it uses it. If not, it creates a new one. Why is this important? Because the more often your DB Server finds a matching plan in the cache, the better it performs. In fact, it can run significantly faster when it is not tasked with constantly building execution plans from scratch. Here's the rub, much of the query code written in Coldfusion requires the RDBMS to compile a new execution plan. Here's why.

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Changing Database Schemas may require a restart of CFMX

Mark Kruger April 29, 2005 4:01 PM SQL tips, Coldfusion & Databases Comments (2)

There's a tricky nuance that you must take into account when you make schema changes in MS SQL. When we discovered the following behavior I looked to see if I could find it blogged or documented somewhere. Failing that, I thought someone else out there might benefit from hearing about this issue. You might run into problems if you are dealing with the following conditions:

  • CFMX using JDBC drivers to MS SQL
  • A view with a "select *" in it for one or more tables
  • The need to change the schema of a particular table referenced with the asterisk (*) within the view
Here's what happens (and how to fix it).

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Using the WITH RECOMPILE option in a Stored Procedure

Mark Kruger April 13, 2005 4:07 PM SQL tips Comments (0)

This is an excellent article by Arthur Fuller that was sent as a newsletter. If you've ever wondered why a stored procedure doesn't save quite as much time as you expected - or why equivelent query code can even be faster - this may be the answer.

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Mark Kruger April 1, 2005 11:17 AM SQL tips, Coldfusion & Databases Comments (1)

(reprinted from a previous blog)

Here's the dilemma. Let's say you have a log table where each row is a record of some action taken by a user. Perhaps the user logs in, updates his profile, searches for products and makes purchase. The table has the following fields:

  • log_id (int) - a primary key for the table.
  • user_id (int) - a foreign key reference to the "users" table.
  • action_type (char) - a string indicating "login", "update", "search" or "purchase".
  • UpdateTime (smalldatetime) - indicates when the event occurred.
Your boss (we'll call him Ralph) comes to you and says "I'd like to know the last action that each user took on the site before they went away. Can you build me a report like that? Well, you know a couple of ways to do this. One is that you can pull in the whole record set ordered by user and updatetime - then track the last record for each user. Or you might pull in all the users ids and do query looking for the max log_id for that user. Both of these approaches will work, but none of them will pull in the data required in a single query. Never Fear, there is another way - it's tricky, but it works!

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the TOP keyword in SQL

Mark Kruger March 18, 2005 3:34 PM SQL tips, Coldfusion & Databases Comments (7)

One of the most common things I see when looking at code I'm trying to optimize is a missunderstanding of how to effectively utilize SQL and a good database platform. Nothing illustrates this more effectively than the constant use of the "maxrow" attribute for a <cfquery> or a <cfoutput> statement. In many cases the TOP keyword should be used instead. Let me Explain...

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