Can you have if-then-else logic in SQL?

Structuring complex conditions

Complex SQL queries can become tangled. By using the CASE statement, you can organize your logic to ensure the data project, customer, or company is prioritized.

SELECT 
  CASE 
    WHEN project_condition THEN project_result
    WHEN customer_condition THEN customer_result
    WHEN company_condition THEN company_result
    ELSE default_result
  END 
FROM data_source;
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Ensure to test various scenarios for your CASE logic, to cover all possible outcomes.

-- While this may look like a traffic light logic in SQL
-- Red: Stop, Think and Code, Green: Run the query, Yellow: Let's Debug!
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Data existence checks

Is the data you need in the table? Use the COUNT() function to verify existence of data before performing intensive operations.

SELECT COUNT(*)
FROM table
WHERE condition;


Should your count return zero, you know to skip the heavy-duty query or maybe it's coffee o'clock! ☕

Merging query results

Run separate SELECT statements and concatenate the outcome with the help of UNION. This allows you to see the „big picture“ from your data.

SELECT column_name FROM table1
WHERE condition1

UNION ALL

SELECT column_name FROM table2
WHERE condition2;


Each SELECT results from different conditions, making your dataset multi-facetted.

-- If the query runs, it's a UNION, if not, it's an ONION (tears may be present)!


Adding flexibility with IF-ELSE

IF-ELSE in MS SQL serves for more sophisticated tasks and resembles control flows of procedural languages:

IF condition
BEGIN
  SELECT 'condition met'
END
ELSE
BEGIN
  SELECT 'default behavior';
END


This construct ensures the engine checks conditions before executing, giving you more control.

-- SQL DB engine: If I could turn back the time ... ops! There is no 'else' in time!
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Top-priority data selections

Choosing the product with the right price can depend on the priority of conditions:

SELECT TOP 1 WITH TIES product, price
FROM products
WHERE condition
ORDER BY priority_column;


The WITH TIES clause ensures that dibs are given to all that tie for top rank.

Making complex logic more digestible

Complex scenarios require breaking down your logic into parts. Using CTEs or subqueries, you can improve the clarity of your code.

Remember, COALESCE() helps select the first non-null value. Finally, BEGIN and END group your IF-ELSE statements into more readable sandwich blocks:

-- Lay it all out in simple chunks, just like your favorite lasagna recipe!


Performance considerations

Performance can become a bottleneck in SQL. Make sure to consider the following:

• Using multiple IF-ELSE expressions can impact performance
• Indexing within CASE should be advanced responsibly
• @@ROWCOUNT checks the number of affected rows
• Prepare a fallback solution when actions are conditional on previous results
• Optimize by limiting column selection and ponder about query simplification or indexing

-- SQL Perf mantra: Faster queries, happier users, happy stress-less life!
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Querying for relevance

Get the most relevant data with a controlled query:

SELECT TOP 1 column_name
FROM table_name
WHERE condition
ORDER BY priority_column;
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This translates into: „bring me the best fitting record, and do it fast!”

Advanced SQL logic tools

Common Table Expressions (CTEs) come in handy when navigating the waters of complex nested IF-THEN-ELSE logic:

WITH OrderedProducts AS (
  SELECT product, price, ROW_NUMBER() OVER (ORDER BY priority DESC) as rn
  FROM products
  WHERE condition
)
SELECT product, price FROM OrderedProducts WHERE rn = 1;
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This gives you the top priority product after ordering all products by priority.

-- CTE is not the newest fitness trend, but it sure does help with logical workouts!
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