Added a SQLite section to query_next_available_episode.md

query_next_available_episode.md

@@ -1,4 +1,4 @@
-# Documentation of the `query query_next_available_episode`
+# Documentation of the query `query_next_available_episode`
 
 ## Purpose
 
@@ -153,6 +153,33 @@ Using the full query against the database on 2023-10-12 we get the following res
 +---------+------------+------------+
 ```
 
+## Differences in the SQLite version
+
+1. Many of the functions used take string arguments, and since the query is
+   stored in a single-quoted string, these have to be escaped with a backslash.
+
+2. The SQLite function to get the current date is `date('now')`
+
+3. The SQLite function to get the day of the week from a date is
+   `strftime('%w',last_date)` and the values are 0 for Sunday and 6 for Saturday.
+
+4. When adding days to a date in SQLite the function is `date(date,'+3 days')`
+   and this is why such strings are returned from the `CASE` statement.
+
+5. To compute the difference in days between two dates in SQLite it is
+   necessary to convert dates to Julian dates and subtract the values. This can
+   give a negative result, so we use `abs()` to prevent it. The result is
+   fractional because of the way Julian dates are implemented in SQLite, so
+   `printf('%i,?)` is used to get the integer part. Therefore, the following
+   expression is needed:
+```
+    printf('%i',
+        abs(
+            julianday(DATE (prev_date, date_offset)) -
+            julianday('now') + 1)
+    ) AS delta_days
+```
+
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