Transforming Multiple Rows into Columns for Each ID in SQLite

Reshaping ID-Associated Names into Separate Columns

Understanding the Data Restructuring Challenge

The core task involves transforming a dataset where each unique identifier (id) is associated with one or two textual entries (name) into a structured format where each identifier occupies a single row, with its corresponding names spread across dedicated columns (name1, name2). This transformation is essential for scenarios requiring denormalized views of data, such as reporting, user interfaces, or data exports.

Example Input:

id  name
4  PRO7
4  ProSieben
5  RTL
6  kabel eins
6  KABEL1
7  RTL II
7  RTL2
8  ARTE

Desired Output:

id  name1      name2
4   PRO7       ProSieben
5   RTL
6   kabel eins KABEL1
7   RTL II     RTL2
8   ARTE

Key Observations:

1. Cardinality Constraint: Each id has a maximum of two associated name values.
2. Order Ambiguity: The sequence of names within each id group (e.g., "PRO7" vs. "ProSieben") is not explicitly defined in the source data. The solution must account for ordering logic (e.g., alphabetical, insertion order).
3. Aggregation Necessity: SQL’s inherent row-based processing requires aggregating or pivoting rows into columns.

Root Causes of Ineffective Row-to-Column Transformations

1. Misapplication of Basic Aggregation Functions

Attempting to use GROUP BY alone fails because it collapses all rows per id into a single row but provides no mechanism to split name values into distinct columns. For example:

SELECT id, name FROM t GROUP BY id;

This returns only the first encountered name per id, discarding subsequent entries.

2. Overlooking Window Functions for Row Numbering

Without assigning a positional index (e.g., first or second name) to each name within its id group, distinguishing name1 from name2 becomes impossible. This necessitates window functions like ROW_NUMBER() to enumerate entries within partitions.

3. Inadequate Handling of Sparse Column Population

When an id has fewer than two names, the result must still include the id with NULL (or empty) values for missing names. Solutions must gracefully handle sparse data without omitting id entries.

4. Misunderstanding Aggregate Function Behavior in Grouped Queries

Using MAX() or MIN() in conjunction with CASE/IIF statements might seem counterintuitive initially. These functions act as filters to isolate the non-NULL value generated by the conditional logic within each id group.

Comprehensive Solutions for Row-to-Column Conversion

1. Leveraging Window Functions and Conditional Aggregation

Step 1: Assign Positional Indexes Using ROW_NUMBER()

Create a derived table that assigns a sequential number (rn) to each name within its id partition. The ORDER BY clause within the window function determines the sequence (e.g., alphabetical order):

SELECT 
  id,
  ROW_NUMBER() OVER (PARTITION BY id ORDER BY name) AS rn,
  name
FROM t;

Intermediate Output:

id  rn  name
4   1   PRO7
4   2   ProSieben
5   1   RTL
6   1   KABEL1
6   2   kabel eins
7   1   RTL II
7   2   RTL2
8   1   ARTE

Step 2: Pivot Rows into Columns Using Conditional Logic

Use CASE expressions (or IIF in SQLite) with MAX() to extract name1 (rn=1) and name2 (rn=2):

SELECT
  id,
  MAX(CASE WHEN rn = 1 THEN name END) AS name1,
  MAX(CASE WHEN rn = 2 THEN name END) AS name2
FROM (
  SELECT 
    id,
    ROW_NUMBER() OVER (PARTITION BY id ORDER BY name) AS rn,
    name
  FROM t
) AS subquery
GROUP BY id
ORDER BY id;

Explanation:

• The inner query assigns rn values.
• CASE WHEN rn = 1 THEN name END generates a column with name for rn=1 and NULL otherwise.
• MAX() ignores NULL values, effectively selecting the sole non-NULL entry per id.

Why MAX() Works:

• Each id group contains exactly one non-NULL value for rn=1 and at most one for rn=2.
• MAX() returns the highest (alphabetically last) value if duplicates exist, but since rn ensures uniqueness, it safely retrieves the intended name.

Alternative Aggregate Functions:

• MIN(): Retrieves the alphabetically first value (if ordering differs).
• GROUP_CONCAT(): Combines multiple values into a string (not applicable here due to cardinality constraint).

2. Utilizing the LEAD() Window Function for Adjacent Row Access

Step 1: Access Subsequent Rows within Partitions

The LEAD() function retrieves the name from the next row within the same id partition:

SELECT
  id,
  name AS name1,
  LEAD(name, 1) OVER (PARTITION BY id ORDER BY name) AS name2,
  ROW_NUMBER() OVER (PARTITION BY id ORDER BY name) AS rn
FROM t;

Intermediate Output:

id  name1       name2        rn
4   PRO7        ProSieben    1
4   ProSieben   NULL         2
5   RTL         NULL         1
6   KABEL1      kabel eins   1
6   kabel eins  NULL         2
7   RTL II      RTL2         1
7   RTL2        NULL         2
8   ARTE        NULL         1

Step 2: Filter to Retain Only the First Row per id

Since name2 is captured in the first row via LEAD(), filter using rn = 1:

SELECT
  id,
  name1,
  name2
FROM (
  SELECT
    id,
    name AS name1,
    LEAD(name, 1) OVER (PARTITION BY id ORDER BY name) AS name2,
    ROW_NUMBER() OVER (PARTITION BY id ORDER BY id) AS rn
  FROM t
) AS subquery
WHERE rn = 1
ORDER BY id;

Advantages:

• Avoids aggregation, which may offer performance benefits on large datasets.
• Directly pairs each name1 with its subsequent name2.

Caveats:

• Requires strict ordering to ensure name1 and name2 align correctly.
• Less intuitive for datasets where id groups exceed two rows.

3. Handling Edge Cases and Optimizations

Scenario 1: Arbitrary Order of Names

If the order of name1 and name2 is irrelevant, omit the ORDER BY clause in the window function:

ROW_NUMBER() OVER (PARTITION BY id) AS rn

Scenario 2: Custom Ordering Logic

To prioritize specific names (e.g., case-insensitive sorting), adjust the ORDER BY:

ROW_NUMBER() OVER (
  PARTITION BY id 
  ORDER BY LOWER(name)
) AS rn

Scenario 3: Expanding Beyond Two Columns

For datasets allowing more than two names per id, extend the pattern:

SELECT
  id,
  MAX(CASE WHEN rn = 1 THEN name END) AS name1,
  MAX(CASE WHEN rn = 2 THEN name END) AS name2,
  MAX(CASE WHEN rn = 3 THEN name END) AS name3
FROM (
  SELECT 
    id,
    ROW_NUMBER() OVER (PARTITION BY id ORDER BY name) AS rn,
    name
  FROM t
) AS subquery
GROUP BY id;

Scenario 4: Dynamic Column Generation

For unknown or variable numbers of names per id, consider application-layer processing or SQLite’s JSON1 extension to serialize names into a JSON array:

SELECT 
  id,
  json_group_array(name) AS names
FROM t
GROUP BY id;

Output:

id  names
4   ["PRO7","ProSieben"]
5   ["RTL"]
6   ["kabel eins","KABEL1"]
7   ["RTL II","RTL2"]
8   ["ARTE"]

4. Performance Considerations

Indexing Strategy:

Create an index on id, name to optimize window function partitioning and ordering:

CREATE INDEX idx_id_name ON t(id, name);

Execution Plan Analysis:

Use EXPLAIN QUERY PLAN to verify that the query leverages indexes and avoids full table scans:

EXPLAIN QUERY PLAN
SELECT ...;

Result Verification:

Cross-validate outputs using subqueries or temporary tables to ensure no id or name is omitted or misplaced during transformation.

Final Query Recommendations:

For static two-name pivots with explicit ordering:

SELECT
  id,
  MAX(IIF(rn = 1, name, NULL)) AS name1,
  MAX(IIF(rn = 2, name, NULL)) AS name2
FROM (
  SELECT 
    id,
    ROW_NUMBER() OVER (PARTITION BY id ORDER BY name) AS rn,
    name
  FROM t
)
GROUP BY id;

For scenarios emphasizing readability and adjacent row access:

SELECT
  id,
  name1,
  name2
FROM (
  SELECT
    id,
    name AS name1,
    LEAD(name) OVER (PARTITION BY id ORDER BY name) AS name2,
    ROW_NUMBER() OVER (PARTITION BY id ORDER BY name) AS rn
  FROM t
)
WHERE rn = 1;

By mastering these techniques, you can efficiently reshape row-based data into columnar formats, enabling clearer insights and streamlined downstream processing.