Course list

Tag: PostgreSQL & SQL

  • Using regexps in PostgreSQL

    After months of writing, editing, and procrastinating, my new ebook, “Practice Makes Regexp” is almost ready.  The book (similar to my earlier ebook, “Practice Makes Python“) contains 50 exercises to improve your fluency with regular expressions (“regexps”), with solutions in Python, Ruby, JavaScript, and PostgreSQL. When I tell people this, they often say, “PostgreSQL?  Really?!?” …

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  • In PostgreSQL, as in life, don’t wait too long to commit

    I recently helped a client with serious PostgreSQL problems. Their database is part of an application that configures and monitors large networks; each individual network node reports its status to the database every five minutes. The client was having problems with one table in particular, containing about 25,000 rows — quite small, by modern database…

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  • Want to understand Python’s comprehensions? Think in Excel or SQL.

    Comprehensions are among the most useful constructs in Python. They merge the old, trusty “map” and “filter” functions into a single piece of compact, elegant syntax, allowing us to expression complex ideas in a minimum of code. Comprehensions are one of the most important tools in a Pythonista’s toolbox. And yet, I have found that…

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  • Remember: Don’t mix = and NULL

    I was recently performing an analysis on the database for my dissertation software, and wanted to look at one particular set of rows.  The table I was looking at is called Nodes, and because each node (represented by a row in the table) can optionally belong to a group, the group_id column allows for NULL…

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  • Turning a PostgreSQL array to rows

    Bottom line: To turn a PostgreSQL array into rows, wrap the column in the UNNEST function: SELECT UNNEST(stuff) FROM foo. Each element of the array becomes its own row, which you can then join, filter, or feed into INTERSECT and UNION. Wrap the whole query in ARRAY(…) to pack the rows back into an array.…

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  • Turning PostgreSQL rows into arrays

    The short version: To turn a set of PostgreSQL rows into an array, wrap a subselect in the ARRAY function: SELECT ARRAY(SELECT description FROM Appointments). It returns one row with one array, holding one element per input row. Wrap that in array_to_string(…, ‘, ‘) to get a comma-separated string, and use AS to give the…

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  • Looking in PostgreSQL arrays with ANY

    So far, this series has looked at how to create PostgreSQL arrays, how to retrieve data from them, and how to get the length of an array’s outer and inner dimensions. But one of the most common actions that we’ll want to do with an array is look inside to see if a particular value…

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  • PostgreSQL array indexes and length

    In my last blog post, I introduced the idea of a PostgreSQL array, and showed how we can insert data into a table using either the curly-brace {} syntax, or the ARRAY construction syntax.  In this post, I want to talk about PostgreSQL indexes and length — what happens when we retrieve from indexes that exist (and…

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  • Learning to love PostgreSQL arrays

    I’ll admit it: When arrays were added to PostgreSQL a number of years ago, I thought that this was a really bad idea.  I’m a firm believer in normalization when it comes to database design and storage; and the idea of putting multiple values inside of a single column struck me as particularly foolish.  Besides,…

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  • Don’t use double quotes in PostgreSQL

    I spend a large proportion of my time teaching classes in a variety of open-source technologies — specifically, Ruby, Python, PostgreSQL, and Git.  One of the questions that invariably arises in these classes has to do with the case sensitivity of the technology in question. That is, is the variable “x” the same as the…

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