andrewlocatelliwoodcock

Thoughts on Software

Posts Tagged ‘Databases

Connecting to Cloudant from Erlang: a quick example of using HTTPS from httpc:request

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Wiser heads than me will no doubt already know this but I for one struggled with working out how to do this so I thought I’d put the example up in the hope it will help others. So, what I was trying to do was get and put to a Cloudant database from Erlang. Cloudant requires both HTTPS and a username and password and I struggled to find an Erlang example online.

First, make sure that you can connect to Cloudant over curl and fetch at least the _all_dbs resource. If you can’t, Erlang isn’t going to work either. I found that to get curl working correctly, I needed to update the ca cert bundle that came with it.

Here’s how to retrive _all_dbs for your Cloudant account using curl:

curl https://username:password@username.cloudant.com/_all_dbs

where username is your Cloudant username and password is the password associated with your username. For example, if your username were ‘foo’ and your password ‘bar’, here’s the command you would use:

curl https://foo:bar@foo.cloudant.com/_all_dbs

If this doesn’t work, the most likely explanation is that your ca cert bundle is out of date: follow the instructions on the curl website for how to update them.
Assuming this worked, your now in a position to try the same with Erlang. I’m going to use the httpc library here:

inets:start(),

ssl:start(),

httpc:request (get, {“https://foo:bar@foo.cloudant.com/_all_dbs”, []}, [{ssl,[{verify,0}]}], []).

It is important to remember to start inets and ssl before attempting to use httpc:request. A put to Cloudant is similar, here’s an example that assumes the existance of a database barfoo and a resource widget on your Cloudant instance and also that you have a suitably initialized variable Update which contains the data which you are writing to the widget resource:

inets:start(),

ssl:start(),

httpc:request (put, {“https://foo:bar@foo.cloudant.com/barfoo/widget”, [], [], Update}, [], []).

And there you have it

Written by andrewlocatelliwoodcock

June 12, 2012 at 11:40

Using SQL and LEFT JOINs to find missing data

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It’s not unusual when working with data-driven applications to be asked to produce ad-hoc exception reoprst of the type: there should a at least one row in TableB for each row in TableA – can you tell me any rows in TableA that do not have at least one corresponding row in TableB?

How do you find something that’s not there?!

Our old friend the LEFT JOIN to the rescue. We already know from this post, that we can use LEF T JOINs to return every row from one table and any matching row from another and this is an extension of the same problem: in this case, we are looking specifically for every row in table A that does not have a matching row in table B.

Here’s how to do it:

SELECT
      a.Id
FROM
      TableA a
LEFT JOIN TableB b ON a.Id = b.Id
WHERE
      b.Id IS NULL

So, we’re returning everything in TableA, anything matching from TableB but then limiting the resultset to only those rows in TableA that DO NOT have a match in TableB. We do that with the statement:

WHERE b.Id IS NULL

This works because the database is returning all rows from TableA and matching rows from TableB but it still has to return something in the case where there are no matching rows in TableB: the special value NULL meaning “unknowable”. What our WHERE clause is saying is “only return those rows in TableA where the matching row in TableB is unknowable”, i.e. where we don’t have a matching row!

Simples.

Written by andrewlocatelliwoodcock

June 4, 2012 at 21:29

Posted in Databases, mySQL, SQL, SQL Server, T-SQL

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How and why to use LEFT JOINs in SQL statements

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LEF T JOINs are something I’ve been using in SQL statements for literally years without thinking much about it but a few conversations recently have made me realize that with the rise of ORMs, a lot of people are a lot less SQL-savvy than they were even a few years ago to the point that JOINs are a bit of a mystery. Most people seem to be able to use INNER JOINs correctly but LEFT JOINs cause a lot of confusion and hence this post …

A LEFT JOIN is used to return data from two tables where there are definitely rows in one table and there may be corresponding rows in the other. If there are, we want to see them and if there aren’t we don’t care: that is, we don’t want to see data only where there is data present in both tables. An example of this could be a report showing all students enrolled in college course and their grades, where some students may not yet have taken any exams but we still want to see all students and then any results for exams they have taken.

So, how do we do this?

Continuing with our example, we’ll need three tables, Students, Courses and Grades with the following schema:

Students: Id, FirstName, SecondName
Courses: Id, Description
Grades: StudentId, CourseId, Grade

What we want to see is all students and every grade they have received over the year. We’ll also want to see the course description so we’ll need to JOIN all three tables. Here’s how we do it:

SELECT 
      s.Id, s.FirstName, s.SecondName, g.Grade, c.Description 
FROM
      Students s
LEFT JOIN Grades g ON s.Id = g.StudentId
INNER JOIN Courses c ON c.Id = g.CourseId

(And just to explain s is declared as an alias of Students so s.Id is the same as writing Students.Id, etc. …)

The LEFT JOIN means: “give me everything on the left of the equals sign and any matching rows on the right of the equals sign”. So

FROM
      Students s
LEFT JOIN Grades g ON s.Id = g.StudentId

means: “give me everything from Students and any matching rows from the Grades table”

So, why the INNER JOIN on Courses? This is what allows us to get the course description from the Courses table. We are assuming here that there should never be a grade for a course that doesn’t exist (pretty reasonable assumption!), so we are restricting the grades we are returning to only those with a matching course.

So there you have it: how to return all students and any matching grades complete with course description in one simple SQL statement.

Later in the week, I’ll be publishing a post on how to use LEFT JOINs to find missing data …

Written by andrewlocatelliwoodcock

May 30, 2012 at 22:11

Posted in Databases, mySQL, SQL, SQL Server

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Creating a table directly from a SELECT statement in mySQL

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It is often useful to be able to select some data we are working with into a new table for further analysis. This is often achieved by first creating the new table and then populating it via a separate SQL statement but it can be quite time-consuming, especially when we want to work with larger numbers of columns, to write the full CREATE TABLE statement for the new table.

What would be ideal would be to be able to infer the structure of the new table directly from the SELECT statement and mySQL does actually give us a way to do that: the CREATE TABLE … SELECT statement.

Here’s an example of how it works:

CREATE TABLE my_new_working_table SELECT column_a, column_b, column_d, column_f FROM my_original_table;

And that’s it: mySQL will create the new table for you, inferring the correct structure from the SELECT statement and also insert the data that matches the SELECT. It won’t add indexes or the like but these can be added afterwards if required and when using this approach, often we only need the table for a short-term analysis task anyway.

Written by andrewlocatelliwoodcock

May 21, 2012 at 21:39

Posted in Databases, mySQL, SQL

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In mySQL, how do you find all tables that have foreign key constraints against another table

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The situation is that I need to know which tables in a database hold a foreign key constraint against a particular table, let’s call it TableX. The reason I need to know this is because I am planning to rename and retire TableX and in mySQL at least, the foreign key contraints follow the table rename. So really, I’m looking for metadata about my database and searching it for references to TableX.

It turns out that this is actually pretty simple:

USE information_schema;
SELECT * FROM KEY_COLUMN_USAGE WHERE REFERENCED_TABLE_NAME = 'TableX' AND TABLE_SCHEMA='[my database name]';

This query will give you all the foreign keys in the named database that reference TableX. To find all foreign keys in the database is even simpler (although I can’t think of a use case for it at the moment):

USE information_schema;
SELECT * FROM KEY_COLUMN_USAGE WHERE TABLE_SCHEMA='bigfishgames';

Simples. Once you know how …

Written by andrewlocatelliwoodcock

May 8, 2012 at 21:53

Posted in Databases, mySQL, SQL

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