SQLite UNION Query Fails with Function in ORDER BY Clause
SQLite UNION Query Behavior with ORDER BY and Functions
When working with SQLite, a common task is to combine results from multiple queries using the UNION operator. However, a specific issue arises when attempting to use a function like lower() in the ORDER BY clause of a UNION query. The query fails with the error: Error: 1st ORDER BY term does not match any column in the result set. This behavior is not a bug but rather a consequence of SQLite’s adherence to standard SQL rules, particularly in the context of UNION queries.
The issue occurs because SQLite enforces strict rules on the ORDER BY clause when used with UNION. Specifically, the ORDER BY clause in a UNION query must reference either a column name from the result set or a positive integer representing the column index. Arbitrary expressions, such as lower(name), are not permitted in this context. This restriction is rooted in the technical limitations of how SQLite processes UNION queries, which differ from how it handles single-table or non-UNION queries.
For example, consider the following query:
SELECT substr(tagname,6) as name FROM tag WHERE tagname GLOB 'wiki-*'
UNION SELECT 'sandbox' as name
ORDER BY lower(name);
This query fails because lower(name) is not a valid expression in the ORDER BY clause of a UNION query. However, replacing lower(name) with name COLLATE NOCASE works as expected:
SELECT substr(tagname,6) as name FROM tag WHERE tagname GLOB 'wiki-*'
UNION SELECT 'sandbox' as name
ORDER BY name COLLATE NOCASE;
This alternative achieves the desired case-insensitive sorting without violating SQLite’s rules for UNION queries.
Standard SQL Restrictions and SQLite’s Technical Limitations
The behavior observed in SQLite is a direct result of its adherence to standard SQL rules, particularly in the context of UNION queries. In standard SQL, the ORDER BY clause in a UNION query must reference either a column name from the result set or a positive integer representing the column index. This restriction ensures that the sorting operation is unambiguous and can be efficiently executed by the database engine.
SQLite extends this rule for non-UNION queries, allowing arbitrary expressions in the ORDER BY clause. However, this extension does not apply to UNION queries due to technical limitations. When processing a UNION query, SQLite combines the results of multiple subqueries into a single result set before applying the ORDER BY clause. This process requires that the ORDER BY clause references only columns or expressions that are explicitly defined in the result set.
For example, consider the following query:
SELECT substr(tagname,6) as name FROM tag WHERE tagname GLOB 'wiki-*'
UNION SELECT 'sandbox' as name
ORDER BY lower(name);
This query fails because lower(name) is not a valid expression in the ORDER BY clause of a UNION query. However, replacing lower(name) with name COLLATE NOCASE works as expected:
SELECT substr(tagname,6) as name FROM tag WHERE tagname GLOB 'wiki-*'
UNION SELECT 'sandbox' as name
ORDER BY name COLLATE NOCASE;
This alternative achieves the desired case-insensitive sorting without violating SQLite’s rules for UNION queries.
Implementing Case-Insensitive Sorting in UNION Queries
To achieve case-insensitive sorting in a UNION query without violating SQLite’s restrictions, you can use the COLLATE NOCASE clause instead of the lower() function. The COLLATE NOCASE clause instructs SQLite to perform a case-insensitive comparison when sorting the result set. This approach is both efficient and compliant with SQLite’s rules for UNION queries.
For example, consider the following query:
SELECT substr(tagname,6) as name FROM tag WHERE tagname GLOB 'wiki-*'
UNION SELECT 'sandbox' as name
ORDER BY name COLLATE NOCASE;
This query successfully sorts the result set in a case-insensitive manner without encountering the Error: 1st ORDER BY term does not match any column in the result set error. The COLLATE NOCASE clause is a powerful tool for achieving case-insensitive sorting in SQLite, particularly in the context of UNION queries.
In addition to using COLLATE NOCASE, you can also achieve case-insensitive sorting by ensuring that the ORDER BY clause references only columns or expressions that are explicitly defined in the result set. For example, you can use the column alias name directly in the ORDER BY clause:
SELECT substr(tagname,6) as name FROM tag WHERE tagname GLOB 'wiki-*'
UNION SELECT 'sandbox' as name
ORDER BY name;
This query sorts the result set based on the name column, which is explicitly defined in the result set. While this approach does not provide case-insensitive sorting, it demonstrates the importance of adhering to SQLite’s rules for UNION queries.
In summary, the issue of using functions like lower() in the ORDER BY clause of a UNION query is a result of SQLite’s adherence to standard SQL rules and its technical limitations. By using the COLLATE NOCASE clause or ensuring that the ORDER BY clause references only columns or expressions explicitly defined in the result set, you can achieve the desired sorting behavior without encountering errors. This approach ensures that your queries are both efficient and compliant with SQLite’s rules for UNION queries.
