UNION Query LIMIT Clause Errors in SQLite Report Generation

Syntax Misapplication in UNION-Combined Batched Queries with Header Rows

The core challenge revolves around generating a report with a header row followed by batched data rows using SQLite’s UNION operator. The header row is intended to originate from a separate table or a specific record, while subsequent batches retrieve 64 records each. A critical syntax error occurs when attempting to combine these elements with LIMIT and OFFSET clauses, resulting in the error: "LIMIT clause should come after UNION NOT before." This error stems from SQLite’s strict parsing rules regarding the placement of LIMIT clauses in compound queries. The ambiguity of which LIMIT clause triggers the error (the one before or after the UNION) complicates debugging efforts. Additionally, the requirement to preserve column consistency between the header and data rows introduces further complexity.

Causes of UNION-LIMIT-OFFSET Syntax Conflicts in Multi-Part Queries

The error arises due to SQLite’s interpretation of LIMIT and OFFSET clauses in the context of compound queries formed with UNION. In standard SQL syntax, the LIMIT and OFFSET clauses apply to the entire compound query unless explicitly scoped to individual SELECT statements. SQLite enforces this by requiring that LIMIT and OFFSET appear after all UNION-connected SELECT statements. When a LIMIT is placed before a UNION (as in SELECT ... LIMIT ... UNION SELECT ...), the parser interprets this as an attempt to apply LIMIT to the first SELECT before combining results with UNION, which violates the clause ordering rules. This is compounded when developers assume that each SELECT in a UNION can independently use LIMIT without encapsulation. The absence of parentheses or subqueries around individual SELECT statements exacerbates the issue, as SQLite treats the entire compound query as a single unit for LIMIT processing.

Another layer of complexity emerges from the requirement to include a header row that structurally matches the data rows. If the header row originates from a different table, column count and type mismatches will trigger additional errors. Even when the header and data rows share the same schema, improper scoping of LIMIT and OFFSET can cause the header row to be excluded or duplicated across batches. Developers often overlook the interaction between UNION’s implicit DISTINCT behavior and the need for precise row ordering, leading to unintended filtering of rows when duplicates exist between the header and data sets.

Resolving UNION-LIMIT Conflicts and Ensuring Header-Data Consistency

Step 1: Encapsulate Individual SELECT Statements with Subqueries
Wrap each SELECT statement in a subquery to isolate their LIMIT and OFFSET clauses. This forces SQLite to process the LIMIT per subquery before applying the UNION:

SELECT * FROM (SELECT * FROM sales LIMIT 1 OFFSET 0)
UNION ALL
SELECT * FROM (SELECT * FROM sales LIMIT 64 OFFSET 1);

The subqueries ensure that the LIMIT clauses apply to their respective SELECT statements independently. Use UNION ALL instead of UNION to prevent implicit deduplication, which is unnecessary when retrieving distinct batches.

Step 2: Validate Column Compatibility Between Header and Data Rows
If the header row originates from a different table, explicitly define columns to ensure consistency:

SELECT 'Region', 'Revenue', 'Units'  -- Static header row
UNION ALL
SELECT region, CAST(revenue AS TEXT), CAST(units AS TEXT) FROM sales LIMIT 64 OFFSET 1;

This approach guarantees column count and type alignment by converting all data columns to TEXT (if necessary) and using string literals for headers. For dynamic headers derived from another table, use a subquery with explicit column aliases:

SELECT * FROM (SELECT region AS header_region, revenue AS header_revenue FROM header_table LIMIT 1)
UNION ALL
SELECT region, revenue FROM sales LIMIT 64 OFFSET 1;

Step 3: Implement Pagination with Dynamic OFFSET Values
For large datasets, generate batches using calculated OFFSET values. Use SQL parameters or application logic to increment OFFSET by batch_size * (page_number - 1) + 1 to skip the header row in subsequent batches:

SELECT * FROM (SELECT * FROM header_table LIMIT 1)
UNION ALL
SELECT * FROM (SELECT * FROM sales LIMIT 64 OFFSET :dynamic_offset);

Replace :dynamic_offset with a value calculated as (page_number - 1) * 64 + 1 to ensure each batch starts after the previous batch’s end, accounting for the initial header row.

Step 4: Address Performance and Indexing for Large Datasets
When using OFFSET on large tables, performance degrades because SQLite must scan all preceding rows. Mitigate this by replacing OFFSET with a WHERE clause filtering on an indexed column:

SELECT * FROM (SELECT * FROM header_table LIMIT 1)
UNION ALL
SELECT * FROM sales WHERE rowid > :last_rowid ORDER BY rowid LIMIT 64;

Maintain :last_rowid in application state to track the highest rowid from the previous batch. This eliminates the need for OFFSET and reduces query execution time.

Step 5: Handle Edge Cases and Validation

• Empty Header Rows: Ensure the header subquery returns exactly one row. Use COALESCE or default values if the header table might be empty.
• Batch Size Mismatches: If a batch returns fewer than 64 rows, handle this in application logic rather than SQL.
• Transaction Isolation: Wrap batched queries in transactions to maintain consistent snapshots of the data during pagination.

By systematically addressing syntax constraints, column compatibility, pagination mechanics, and performance optimizations, developers can robustly generate batched reports with header rows in SQLite while avoiding common UNION–LIMIT pitfalls.