How to Correctly Calculate Aggregates Using SQL LEFT JOINS and IF Statements.

Understanding SQL LEFT JOINS and Grouping by Multiple Columns

In this article, we will explore the concept of SQL LEFT JOINs and how to group data using multiple columns.

Introduction to SQL LEFT JOINs

A LEFT JOIN (also known as a LEFT OUTER JOIN) is used to combine rows from two tables based on a related column between them. It returns all rows from the left table and matching rows from the right table, if any exist. If there are no matches, the result set will contain NULL values for the right table columns.

Example: Incorrect Result with LEFT JOIN

Our example query uses four tables: TH_STOCK_LEVELS, TH_STOCK_ITEMS, TH_ADVICE_NOTE_LINES, and TH_HIRE_ITEMS. We use a LEFT JOIN to combine data from these tables, but we encounter an issue where the AdviceQty and HireQty columns are not calculated correctly.

SELECT 
    i.SKI_NAME AS Description,
    l.SKV_QUANTITY_IN_STOCK AS Qty,
    SUM(CASE WHEN ad.AVN_STATUS = 'D' THEN li.AVL_QUANTITY ELSE '0' END) AS AdviceQty,
    SUM(CASE WHEN con.HCT_STATUS = 'D' THEN item.HIT_QUANTITY ELSE '0' END) AS HireQty
    
FROM 
    TH_STOCK_LEVELS l
    
LEFT JOIN TH_STOCK_ITEMS i ON l.SKV_STOCK_NUMBER = i.SKI_STOCK_NUMBER
LEFT JOIN TH_ADVICE_NOTE_LINES li ON i.SKI_NAME = li.AVL_DESCRIPTION
LEFT JOIN TH_HIRE_ITEMS item ON li.AVL_DESCRIPTION = item.HIT_DESCRIPTION
LEFT JOIN TH_ADVICE_NOTES ad ON ad.AVN_ID = li.AVL_NOTE_NUMBER
LEFT JOIN TH_HIRE_CONTRACTS con ON con.HCT_CONTRACT_NUMBER = item.HIT_CONTRACT_NUMBER
    
WHERE 
    l.SKV_DEPOT_ID = 7
    
GROUP BY i.SKI_NAME, l.SKV_QUANTITY_IN_STOCK;

The query returns all rows from the TH_STOCK_LEVELS table and matching rows from the other tables, but it incorrectly calculates the AdviceQty and HireQty columns.

Solution: Using IF Statements with LEFT JOINs

To solve this issue, we can use IF statements instead of CASE statements. The IF statement allows us to check for conditions and return different values based on those conditions.

SELECT 
    i.SKI_NAME AS Description,
    l.SKV_QUANTITY_IN_STOCK AS Qty,
    SUM(IF(ad.AVN_STATUS='D',li.AVL_QUANTITY,0)) AS AdviceQty,
    SUM(IF(con.HCT_STATUS='D',item.HIT_QUANTITY,0)) AS HireQty
    
FROM 
    TH_STOCK_LEVELS l
    
LEFT JOIN TH_STOCK_ITEMS i ON l.SKV_STOCK_NUMBER = i.SKI_NAME
LEFT JOIN TH_ADVICE_NOTE_LINES li ON i.SKI_NAME = li.AVL_DESCRIPTION
LEFT JOIN TH_HIRE_ITEMS item ON li.AVL_DESCRIPTION = item.HIT_DESCRIPTION
LEFT JOIN TH_ADVICE_NOTES ad ON ad.AVN_ID = li.AVL_NOTE_NUMBER
LEFT JOIN TH_HIRE_CONTRACTS con ON con.HCT_CONTRACT_NUMBER = item.HIT_CONTRACT_NUMBER
    
WHERE 
    l.SKV_DEPOT_ID = 7
    
GROUP BY i.SKI_NAME, l.SKV_QUANTITY_IN_STOCK;

In this revised query, we use IF statements to check if the AVN_STATUS column in the TH_ADVICE_NOTES table is ‘D’ or not. If it’s ‘D’, we return the li.AVL_QUANTITY value; otherwise, we return 0.

Why IF Statements Work

The IF statement works because SQL allows us to use a condition and return different values based on that condition. In this case, we check if the AVN_STATUS column is ‘D’ and return the li.AVL_QUANTITY value if it’s true.

Conclusion

In conclusion, LEFT JOINs can be used to combine data from multiple tables, but they require careful consideration of how to calculate aggregates (like AdviceQty and HireQty) based on specific conditions. By using IF statements instead of CASE statements, we can solve this issue and return the correct results for our query.

Best Practices

When working with LEFT JOINs, remember to:

Always specify the tables you want to combine
Use the correct join condition
Consider how to calculate aggregates based on specific conditions
Use IF statements instead of CASE statements when necessary

Last modified on 2024-06-28