Added query_next_available_episode.md and updated templates.md
Dave Morriss
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query_next_available_episode.md
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query_next_available_episode.md
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# Documentation of the `query query_next_available_episode`
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## Purpose
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This query finds the next free slot in the database. It starts from the slot with the latest date and looks forward for the next numeric slot that is empty. The query returns the number of the free slot found, its date, and the number of days from the current date the date of the free slot.
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The date difference is used in the template `content-index.tpl.html` which generates the *Home* or *Index* page on the static site.
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## Overview
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There are two versions of the query, for MySQL and SQLite databases. The queries are held (with other queries) in the files `queries-index-mysql.tpl.html` and `queries-index-sqlite.tpl.html`.
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## Query analysis
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The query contains two CTEs (Common Table Expressions). These are a relatively new addition to the SQL standard (added in 2005). They can be regarded as temporary *views* which can be used in other parts of the query.
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CTEs are introduced by the `WITH` statement:
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```
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WITH cte_1 (CTE definition 1),
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cte_2 (CTE definition 2)
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SELECT
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main query;
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```
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### First CTE (`next_id`)
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This is:
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```
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WITH next_id AS (
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SELECT id, id + 1 AS id_next, date as last_date
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FROM eps
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WHERE eps.date > NOW()
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),
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```
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It is a simple `SELECT` on the `eps` table looking at all future shows from the current date onwards. This is the MySQL variant. It can be run stand-alone to see what it does, but because the MySQL database doesn't return episodes in `id` order it's necessary to use:
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```
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SELECT id, id + 1 AS id_next, date as last_date
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FROM eps
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WHERE eps.date > NOW()
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order by id limit 5;
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```
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This (when run on 2023-10-12) gives (using only 5 rows for brevity):
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```
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+------+---------+------------+
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| id | id_next | last_date |
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+------+---------+------------+
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| 3965 | 3966 | 2023-10-13 |
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| 3970 | 3971 | 2023-10-20 |
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| 3971 | 3972 | 2023-10-23 |
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| 3973 | 3974 | 2023-10-25 |
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| 3980 | 3981 | 2023-11-03 |
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+------+---------+------------+
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```
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This is the view-like or table-like information which will be available to the rest of the query. Note that we see allocated episode numbers and that number plus 1 for each row. The former exists in the table, while the latter may not. This is done because it's not possible to look up an non-existent row!
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### Second CTE (`free_slot`)
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This CTE uses the output from the first one (`next_id`) as well as the contents
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of the `eps` table:
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```
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free_slot AS (
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SELECT
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last_date AS prev_date,
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CASE
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WHEN DAYOFWEEK(last_date) = 6
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THEN 3
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ELSE 1
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END AS date_offset,
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next_id.id_next AS empty_slot
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FROM next_id
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LEFT JOIN eps ON next_id.id_next = eps.id
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WHERE eps.id IS NULL
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ORDER BY next_id.id
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LIMIT 1
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)
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```
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Here the query is using `SELECT` on the rows from the first CTE (`next_id`) and joining them with the `eps` table, using `next_id.id_next` as the joining criterion. A `LEFT JOIN` is used because it reveals null values.
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If the `eps.id` value is `NULL` it means there is no matching row in the `eps` table. In other words, a free slot has been found.
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The ordering of the `eps_id` column is done in this CTE because of the lack of order in the table and because it's simpler to add it here than in the other CTE.
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We are only interested in the first row that matches the criteria, so a `LIMIT 1` is used.
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- The `prev_date` field returned will hold the date of the episode before the empty slot.
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- The `CASE` expression computes the day of the week of this date, which in
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MySQL is 1 for Sunday and 7 for Saturday. We are interested in whether it's a Friday because we need to return the offset needed to reach the next weekday. This will be 3 days for Friday and 1 day for other days. We trust that the `prev_date` will never fall on a weekend. What is returned is a field called `date_offset` containing either a 3 or a 1.
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- The `empty_slot` field uses the `next_id.id_next` value which is the number
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of the episode that doesn't exist yet.
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Testing these two CTEs with a simple main query we see:
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```
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WITH next_id AS (
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SELECT id, id + 1 AS id_next, date as last_date
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FROM eps
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WHERE eps.date > NOW()
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),
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free_slot AS (
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SELECT
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last_date AS prev_date,
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CASE
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WHEN DAYOFWEEK(last_date) = 6
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THEN 3
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ELSE 1
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END AS date_offset,
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next_id.id_next AS empty_slot
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FROM next_id
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LEFT JOIN eps ON next_id.id_next = eps.id
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WHERE eps.id IS NULL
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ORDER BY next_id.id
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LIMIT 1
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)
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SELECT * FROM free_slot;
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```
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This produces the output:
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```
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+------------+-------------+------------+
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| prev_date | date_offset | empty_slot |
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+------------+-------------+------------+
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| 2023-10-13 | 3 | 3966 |
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+------------+-------------+------------+
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```
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This shows the date of the last show before the empty slot, the number of days to offset it and the number of the free slot.
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### Main query
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Finally, the main query makes use of the three values returned from the two CTEs:
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```
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SELECT
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empty_slot AS next_id,
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ADDDATE(prev_date,date_offset) AS next_date,
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DATEDIFF(ADDDATE(prev_date,date_offset),NOW()) AS delta_days
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FROM free_slot
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```
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This query computes the values actually needed by the template that invokes it:
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- Field `next_id` contains the number of the empty slot.
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- Field `next_date` contains the date of the empty slot. This is computed by
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adding 3 days to the date from the CTEs (in this example on 2023-10-12).
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- Field `delta_days` is computed by subtracting the current date from the
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future `next_date` value using `DATEDIFF`.
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Using the full query against the database on 2023-10-12 we get the following result:
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```
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+---------+------------+------------+
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| next_id | next_date | delta_days |
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+---------+------------+------------+
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| 3966 | 2023-10-16 | 4 |
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+---------+------------+------------+
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```
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<!--
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vim: syntax=pandoc:ts=8:sw=4:tw=0:et:ai:
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-->
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@ -334,8 +334,8 @@ TBA
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## queries-index-mysql.tpl.html
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- Contains several database queries for MySQL assigned to `TT²` variables:
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- `query_next_available_episode` - computes the number of days to the next
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free slot, using two CTEs
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- [`query_next_available_episode`](query_next_available_episode) -
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computes the number of days to the next free slot, using two CTEs
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- `query_latest_episodes` - collects the latest 10 shows, hosts, series
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and comment counts for the main index. Uses a CTE for the comment
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counts.
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Reference in New Issue
Block a user