SQL Queries

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  1. SQL Queries: A Beginner's Guide

This article provides a comprehensive introduction to SQL (Structured Query Language) queries, tailored for beginners working within a MediaWiki environment (version 1.40 and later). SQL is the standard language for accessing and manipulating data stored in relational database management systems (RDBMS). Understanding SQL is crucial for anyone needing to extract, modify, or analyze data, and is particularly relevant for MediaWiki administrators, developers, and power users who wish to work directly with the wiki's database.

What is SQL?

SQL is *not* a programming language in the traditional sense. It's a declarative language, meaning you tell the database *what* data you want, not *how* to get it. The database engine then figures out the most efficient way to retrieve the information. This contrasts with procedural languages, where you specify a step-by-step process.

Think of it like ordering food at a restaurant. You tell the waiter (the SQL engine) *what* you want (the data), and the kitchen (the database) prepares it for you. You don't need to know how the chef cooks the meal.

Basic SQL Syntax

SQL queries are built using a set of keywords and operators. Here's a breakdown of the fundamental components:

  • **SELECT:** Specifies the columns you want to retrieve.
  • **FROM:** Specifies the table from which to retrieve the data.
  • **WHERE:** Filters the data based on specified conditions.
  • **ORDER BY:** Sorts the results.
  • **GROUP BY:** Groups rows that have the same values in specified columns.
  • **HAVING:** Filters groups based on specified conditions (used with `GROUP BY`).
  • **LIMIT:** Restricts the number of rows returned.

A simple SQL query looks like this:

```sql SELECT column1, column2 FROM table_name WHERE condition; ```

Let's break it down:

  • `SELECT column1, column2`: This part tells the database to retrieve the values from the columns named `column1` and `column2`. You can use `SELECT *` to retrieve all columns from the table.
  • `FROM table_name`: This specifies that the data should be pulled from a table called `table_name`.
  • `WHERE condition`: This filters the results. Only rows that meet the specified `condition` will be included in the output.

Core SQL Queries

Let's explore some common SQL queries with examples focusing on a hypothetical MediaWiki database. Assume we have tables like `user`, `page`, `revision`, and `category`. See MediaWiki database schema for further details.

      1. 1. SELECT Queries

The most basic query.

```sql SELECT user_name FROM user WHERE user_id = 5; ```

This query retrieves the `user_name` from the `user` table where the `user_id` is equal to 5.

To select all columns:

```sql SELECT * FROM page WHERE page_title = 'Main Page'; ```

This retrieves all information about the page titled 'Main Page'.

      1. 2. Filtering with WHERE Clause

The `WHERE` clause is essential for extracting specific data.

  • **Equality:** `WHERE column = 'value'`
  • **Inequality:** `WHERE column != 'value'` or `WHERE column <> 'value'`
  • **Greater Than:** `WHERE column > 10`
  • **Less Than:** `WHERE column < 10`
  • **Greater Than or Equal To:** `WHERE column >= 10`
  • **Less Than or Equal To:** `WHERE column <= 10`
  • **Between:** `WHERE column BETWEEN 10 AND 20`
  • **Like:** `WHERE column LIKE '%pattern%'` (for pattern matching – see SQL LIKE operator)
  • **In:** `WHERE column IN ('value1', 'value2', 'value3')`
  • **Is NULL:** `WHERE column IS NULL`
  • **Is NOT NULL:** `WHERE column IS NOT NULL`

Example:

```sql SELECT page_title FROM page WHERE page_namespace = 0 AND page_len > 1000; ```

This retrieves the titles of all pages in the main namespace (namespace 0) that have more than 1000 bytes of content.

      1. 3. Sorting with ORDER BY Clause

The `ORDER BY` clause sorts the results.

```sql SELECT page_title FROM page ORDER BY page_title ASC; -- Ascending order (default) SELECT page_title FROM page ORDER BY page_title DESC; -- Descending order ```

This sorts the page titles alphabetically (ascending) and reverse alphabetically (descending). You can also order by multiple columns:

```sql SELECT page_title, page_len FROM page ORDER BY page_len DESC, page_title ASC; ```

This sorts by page length (longest first) and then, for pages with the same length, by title (alphabetically).

      1. 4. Limiting Results with LIMIT Clause

The `LIMIT` clause restricts the number of rows returned.

```sql SELECT page_title FROM page LIMIT 10; ```

This retrieves the first 10 page titles.

```sql SELECT page_title FROM page LIMIT 5 OFFSET 10; ```

This retrieves 5 page titles starting from the 11th row (offset 10). Useful for pagination.

      1. 5. Aggregate Functions

Aggregate functions perform calculations on a set of rows and return a single value.

  • **COUNT():** Counts the number of rows.
  • **SUM():** Sums the values in a column.
  • **AVG():** Calculates the average value in a column.
  • **MIN():** Finds the minimum value in a column.
  • **MAX():** Finds the maximum value in a column.

Example:

```sql SELECT COUNT(*) FROM user; ```

This counts the total number of users in the `user` table.

```sql SELECT AVG(page_len) FROM page; ```

This calculates the average page length.

      1. 6. Grouping with GROUP BY Clause

The `GROUP BY` clause groups rows that have the same values in specified columns. It's often used with aggregate functions.

```sql SELECT page_namespace, COUNT(*) FROM page GROUP BY page_namespace; ```

This counts the number of pages in each namespace.

      1. 7. Filtering Groups with HAVING Clause

The `HAVING` clause filters groups based on specified conditions. It's used with `GROUP BY`.

```sql SELECT page_namespace, COUNT(*) FROM page GROUP BY page_namespace HAVING COUNT(*) > 100; ```

This counts the number of pages in each namespace, but only includes namespaces that have more than 100 pages.

      1. 8. Joining Tables

Joining tables combines data from multiple tables based on a related column. The most common type of join is the `INNER JOIN`.

```sql SELECT user_name, page_title FROM user INNER JOIN revision ON user.user_id = revision.rev_user INNER JOIN page ON revision.rev_page = page.page_id WHERE page_title = 'Main Page'; ```

This query retrieves the user name and page title for all revisions made to the 'Main Page'. It joins the `user`, `revision`, and `page` tables based on their respective IDs. Understanding database joins is critical for complex queries.

Advanced SQL Concepts

  • **Subqueries:** Queries nested within other queries.
  • **Views:** Virtual tables based on the result of a query.
  • **Stored Procedures:** Precompiled SQL code that can be executed repeatedly.
  • **Transactions:** A set of SQL statements that are treated as a single unit of work.
  • **Indexes:** Data structures that speed up data retrieval.

SQL and MediaWiki

MediaWiki uses MySQL/MariaDB as its database. Therefore, the SQL syntax described above is applicable. However, directly querying the MediaWiki database can be dangerous if not done carefully. Always back up your database before making any changes. Consider using the MediaWiki API for safer data access.

Tips for Writing Effective SQL Queries

  • **Use meaningful aliases:** Make your queries easier to read and understand.
  • **Format your code:** Use indentation and capitalization to improve readability.
  • **Test your queries:** Verify that your queries return the expected results.
  • **Use indexes:** Improve query performance.
  • **Avoid `SELECT *`:** Only select the columns you need.
  • **Be careful with `LIKE`:** Leading wildcards (`%pattern`) can significantly slow down queries.

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