SQLite UNION ALL Sorting Issue with CASE in ORDER BY Clause

When working with SQLite, particularly when combining multiple result sets using UNION ALL, sorting the combined results can sometimes lead to unexpected behavior, especially when attempting to use a CASE statement within the ORDER BY clause. The core issue arises when the ORDER BY clause references a column that is not directly present in the combined result set, or when it attempts to apply transformations to a column that is present. This issue is particularly prevalent when dealing with JSON data and attempting to sort based on specific JSON keys or paths.

The problem manifests when the ORDER BY clause attempts to sort the combined result set using a CASE statement that references a column that is not directly part of the output. SQLite requires that the ORDER BY clause only references columns that are explicitly part of the result set. This requirement is due to the way SQLite internally handles the UNION ALL operation, which involves creating a temporary table to store the combined results. The temporary table only includes the columns that are explicitly selected in the SELECT statements, and any transformations or calculations applied to these columns are not automatically included.

Interrupted Write Operations Leading to Index Corruption

The root cause of this issue lies in the way SQLite handles the UNION ALL operation and the subsequent sorting of the combined result set. When you use UNION ALL, SQLite combines the results of multiple SELECT statements into a single result set. This combined result set is then sorted based on the ORDER BY clause. However, SQLite imposes a restriction that the ORDER BY clause can only reference columns that are explicitly part of the combined result set. This restriction is in place because SQLite creates a temporary table to store the combined results, and this temporary table only includes the columns that are explicitly selected in the SELECT statements.

In the case of the CASE statement within the ORDER BY clause, the issue arises because the CASE statement attempts to reference a column that is not directly part of the combined result set. Even though the column is present in the individual SELECT statements, the CASE statement is not able to reference it directly because it is not part of the combined result set. This leads to the error message: "2nd ORDER BY term does not match any column in the result set."

Another contributing factor is the way SQLite handles JSON data. When working with JSON data, the json_each and json_tree functions are used to extract specific elements from the JSON structure. These functions return a set of rows, each containing the extracted data. However, the columns returned by these functions are not automatically included in the combined result set unless they are explicitly selected. This can lead to confusion when attempting to sort the combined result set based on JSON keys or paths.

Implementing CASE in SELECT and Subselects for Proper Sorting

To resolve this issue, there are several approaches that can be taken. The most straightforward approach is to include the CASE statement directly in the SELECT statements that make up the UNION ALL operation. By doing this, the result of the CASE statement becomes part of the combined result set, and can then be referenced in the ORDER BY clause.

For example, consider the following query:

select json_array_length(value) as v, 1 as ro, fullkey as fk, 
       case fullkey when '$.r' then 1 when '$.c' then 2 else 3 end as ordering
from json_each('{"r":"D","c":"E","d":[ { "n": "name_1", "t": "d" }, { "n": "name_2", "t": "d" }, { "n": "name_3", "t": "f" },{ "n": "name_4", "t": "f" },{ "n": "name_5", "t": "f" }]}')
where fullkey = '$.d'
union all
select value as v, 2 as ro, fullkey as fk, 
       case fullkey when '$.r' then 1 when '$.c' then 2 else 3 end as ordering
from json_tree('{"r":"D","c":"E","d":[ { "n": "name_1", "t": "d" }, { "n": "name_2", "t": "d" }, { "n": "name_3", "t": "f" },{ "n": "name_4", "t": "f" },{ "n": "name_5", "t": "f" }]}')
where fullkey in ('$.r', '$.c') or path = '$.d'
order by ro, ordering;

In this query, the CASE statement is included in both SELECT statements, and the result is aliased as ordering. This column is then referenced in the ORDER BY clause, allowing the combined result set to be sorted correctly.

Another approach is to use a subselect or a Common Table Expression (CTE) to first combine the results and then apply the ORDER BY clause. This approach allows the CASE statement to be applied to the combined result set, rather than to the individual SELECT statements. For example:

with json(json) as (
  values ('{"r":"D","c":"E","d":[ { "n": "name_1", "t": "d" }, { "n": "name_2", "t": "d" }, { "n": "name_3", "t": "f" },{ "n": "name_4", "t": "f" },{ "n": "name_5", "t": "f" }]}')
),
data(v, ro, fk) as (
  select json_array_length(value), 1, fullkey
  from json, json_each(json.json)
  where fullkey = '$.d'
  union all
  select value, 2, fullkey
  from json, json_tree(json.json)
  where fullkey in ('$.r', '$.c') or path = '$.d'
)
select v, ro, fk
from data
order by ro, case fk when '$.r' then 1 when '$.c' then 2 else 3 end;

In this query, the json CTE is used to define the JSON data, and the data CTE is used to combine the results of the json_each and json_tree functions. The ORDER BY clause is then applied to the combined result set, allowing the CASE statement to be used to sort the results.

Both of these approaches ensure that the CASE statement is applied to the combined result set, rather than to the individual SELECT statements. This allows the ORDER BY clause to reference the result of the CASE statement, and ensures that the combined result set is sorted correctly.

In conclusion, when working with SQLite and attempting to sort a combined result set using a CASE statement within the ORDER BY clause, it is important to ensure that the CASE statement is applied to the combined result set, rather than to the individual SELECT statements. This can be achieved by including the CASE statement directly in the SELECT statements, or by using a subselect or CTE to first combine the results and then apply the ORDER BY clause. By following these approaches, you can ensure that your combined result set is sorted correctly, even when dealing with complex JSON data.

SQLite UNION ALL Sorting Issue with CASE in ORDER BY Clause