Monday, January 8, 2018

Common Table Expressions (CTEs) Part 1

Occasionally at conference or a Meetup, someone will approach me and ask me for help with a MySQL problem.  Eight out of ten times their difficulty includes a sub query. "I get an error message about a corrugated or conflabugated sub query or some such,"  they say, desperate for help.  Usually with a bit of fumbling we can get their problem solved.  The problem is not a lack of knowledge for either of us but that sub queries are often hard to write. 

MySQL 8 will be the first version of the most popular database on the web with Common Table Expressions or CTEs.  CTEs are a way to create temporary tables and then use that temporary table for queries. Think of them as easy to write sub queries!

WITH is The Magic Word

The new CTE magic is indicated with the WITH clause.

mysql> WITH myfirstCTE 
      AS (SELECT * FROM world.city WHERE CountryCode='USA')
SELECT Name, District
FROM myfirstCTE 
ORDER BY Name
LIMIT 5;
+-------------+------------+
| Name        | District   |
+-------------+------------+
| Abilene     | Texas      |
| Akron       | Ohio       |
| Albany      | New York   |
| Albuquerque | New Mexico |
| Alexandria  | Virginia   |
+-------------+------------+
5 rows in set (0.01 sec)

mysql> 

So in the above example we create at temporary table named 'myfirstCTE' and then query from that table. The CTE itself isa very simple query, SELECT * FROM world.city. And then two columns are plucked out of 'myfirstCTE'. The CTE can even have modifiers on the query for ORDER BY, LIMIT or any other SQL qualifier.

A Little More Complex Example 


Lets try a little more complex query.

mysql> WITH secondCTE AS 
     (SELECT city.Name AS A, country.Name as B, city.District AS C 
     FROM city 
     JOIN country on (city.CountryCode=country.Code)) 
     SELECT A, B, C FROM secondCTE LIMIT 5;
+----------------+-------------+----------+
| A              | B           | C        |
+----------------+-------------+----------+
| Oranjestad     | Aruba       | –        |
| Kabul          | Afghanistan | Kabol    |
| Qandahar       | Afghanistan | Qandahar |
| Herat          | Afghanistan | Herat    |
| Mazar-e-Sharif | Afghanistan | Balkh    |
+----------------+-------------+----------+
5 rows in set (0.04 sec)

mysql> 

Here we select columns A, B, C from temporary table 'secondCTE'. In this case it is easier to see that we are getting data from the CTE and not the two tables city or country.  At least directly.

The reason I use this examples is often times you have to join two or more tables and the columns with the name name have different sources, here name of country and name of city.  Yes, you could alias them in a sub query but the legibility of the query increases.  And as query complexity builds things like the old N+1 problems raise their ugly heads.

More Than One Way to Skin a Cat 

So lets play again with the information from the second example but this time we will write two separate CTEs and then join them together.  First we are going to get the Name column from the city table and the Name column from the country table. We do have to drag along the columns for a join in the CTEs.  I think for many with limited SQL experience that this version is a little easier to comprehend than the second example.

mysql> WITH threeA AS (SELECT Name AS N1, CountryCode AS x FROM city),
    -> threeB AS (SELECT Name AS N2, Code as y FROM country)
    -> SELECT N1, N2 FROM threeA JOIN threeB WHERE threeA.x = threeB.y LIMIT 5;
+----------------+-------------+
| N1             | N2          |
+----------------+-------------+
| Oranjestad     | Aruba       |
| Kabul          | Afghanistan |
| Qandahar       | Afghanistan |
| Herat          | Afghanistan |
| Mazar-e-Sharif | Afghanistan |
+----------------+-------------+
5 rows in set (0.01 sec)

mysql> 
 
Hopefully the use of CTEs will produce easier to read SQL statements. 

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