Joining Multiple Tables with a Left Join

In this article, we will explore how to join multiple tables using a left join. We will also delve into the nuances of left joins and how to use them effectively in SQL queries.

Understanding Left Joins

A left join is a type of join that returns all records from the left table (e), and the matched records from the right tables (c and o). The result set contains NULL values for the right columns where there are no matches.

The syntax for a left join is as follows:

SELECT column1, column2
FROM left_table
LEFT JOIN right_table1 ON condition1
LEFT JOIN right_table2 ON condition2;

In our example, we want to join Events (e), Contracts (c), and Occurences (o) on the other_id column.

Grouping by guid

We need to group the results by guid, as there may be multiple records for each guide. To do this, we can add a GROUP BY clause:

SELECT e.id, e.date, c.isHappy, c.user, o.interactions
FROM Events e
LEFT JOIN Contracts c ON e.other_id = c.other_id
LEFT JOIN Occurences o ON e.guid = o.guid
WHERE e.eventType = 'reject'
  AND e.date >= '2024-11-11'
GROUP BY e.guid;

However, this will not return the latest record for each guide. We need to modify our query to use a subquery or a Common Table Expression (CTE) to get the latest record for each guide.

Using a Subquery

We can use a subquery to get the latest record for each guide:

SELECT e.id, e.date, c.isHappy, c.user, o.interactions
FROM Events e
LEFT JOIN Contracts c ON e.other_id = c.other_id
LEFT JOIN (
  SELECT guid, MAX(id) AS max_id
  FROM Occurences
  WHERE date >= '2024-11-11'
  GROUP BY guid
) o ON e.guid = o.guid AND e.id = o.max_id
WHERE e.eventType = 'reject';

In this query, the subquery gets the maximum id for each guide in the Occurences table where the date is greater than or equal to 2024-11-11. The outer query then joins Events and Contracts on the other_id column and selects the records where the guid matches and the id is equal to the maximum id for that guide.

Using a Common Table Expression (CTE)

Alternatively, we can use a CTE to get the latest record for each guide:

WITH latest_records AS (
  SELECT guid, MAX(id) AS max_id
  FROM Occurences
  WHERE date >= '2024-11-11'
  GROUP BY guid
)
SELECT e.id, e.date, c.isHappy, c.user, o.interactions
FROM Events e
LEFT JOIN Contracts c ON e.other_id = c.other_id
LEFT JOIN latest_records o ON e.guid = o.guid AND e.id = o.max_id
WHERE e.eventType = 'reject';

In this query, the CTE gets the maximum id for each guide in the Occurences table where the date is greater than or equal to 2024-11-11. The outer query then joins Events and Contracts on the other_id column and selects the records where the guid matches and the id is equal to the maximum id for that guide.

Handling NULL Values

In our query, we are handling NULL values by using the LEFT JOIN keyword. However, if there are no matches in the right table, the result set will contain NULL values for those columns.

To handle this situation, we can use the COALESCE function to replace NULL values with a default value:

SELECT e.id, e.date, c.isHappy, COALESCE(c.user, 'Unknown') AS user, o.interactions
FROM Events e
LEFT JOIN Contracts c ON e.other_id = c.other_id
LEFT JOIN Occurences o ON e.guid = o.guid AND e.id = (SELECT MAX(id) FROM Occurences WHERE guid = o.guid AND date >= '2024-11-11')
WHERE e.eventType = 'reject';

In this query, the COALESCE function is used to replace NULL values in the user column with the string 'Unknown'.

Conclusion

Joining multiple tables using a left join requires careful planning and attention to detail. By understanding how left joins work and using techniques such as grouping by guid, subqueries, and CTEs, we can effectively join multiple tables and handle NULL values.

In this article, we have explored how to join multiple tables with a left join and how to use techniques such as grouping by guid and handling NULL values. We hope that this article has provided you with the knowledge and skills necessary to tackle complex SQL queries.

Last modified on 2023-10-08