Part 5 – Filtering Healthcare Data with WHERE and HAVING

In this project, I’m learning how to filter data effectively using the WHERE and HAVING clauses. Instead of working with the entire dataset, these techniques allow me to focus only on the specific information I need for analysis.

What You’ll Learn

• How to filter individual rows using WHERE
• How to combine multiple conditions with AND, OR, IN, BETWEEN, and LIKE
• How to filter grouped results using HAVING
• Practical examples using the Medicare Hospital Readmission dataset

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1. Filtering with WHERE

The WHERE clause is used to filter rows before any grouping happens.

-- Example: Filter hospitals in California only
SELECT 
    Facility_Name,
    State,
    Predicted_Readmission_Rate
FROM Hospital_Readmissions
WHERE State = 'CA'
ORDER BY Predicted_Readmission_Rate DESC;

2. Using AND / OR

You can combine multiple conditions using AND and OR.

-- Hospitals in California with readmission rate over 20%
SELECT 
    Facility_Name,
    State,
    Predicted_Readmission_Rate
FROM Hospital_Readmissions
WHERE State = 'CA'
  AND Predicted_Readmission_Rate > 20
ORDER BY Predicted_Readmission_Rate DESC;

3. Using IN

The IN operator is useful when you want to filter by multiple specific values.

-- Example: Show hospitals in CA, NY, or TX
SELECT 
    Facility_Name,
    State,
    Predicted_Readmission_Rate
FROM Hospital_Readmissions
WHERE State IN ('CA', 'NY', 'TX')
ORDER BY Predicted_Readmission_Rate DESC;

4. Using BETWEEN

BETWEEN is used to filter within a range of values.

-- Example: Hospitals with readmission rates between 15 and 25
SELECT 
    Facility_Name,
    State,
    Predicted_Readmission_Rate
FROM Hospital_Readmissions
WHERE Predicted_Readmission_Rate BETWEEN 15 AND 25
ORDER BY Predicted_Readmission_Rate DESC;

5. Using LIKE (Pattern Matching)

LIKE is used for partial text matching. It is helpful when you don’t know the exact value.

-- Example: Find hospitals containing "Medical Center" in the name
SELECT 
    Facility_Name,
    State
FROM Hospital_Readmissions
WHERE Facility_Name LIKE '%Medical Center%'
ORDER BY Predicted_Readmission_Rate DESC;

6. Filtering with HAVING

While WHERE filters rows before grouping, HAVING filters after grouping. It is commonly used with aggregate functions.

-- Example: States with more than 500 hospitals
SELECT 
    State,
    COUNT(DISTINCT Facility_ID) AS Total_Hospitals
FROM Hospital_Readmissions
GROUP BY State
HAVING COUNT(DISTINCT Facility_ID) > 500
ORDER BY Total_Hospitals DESC;

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Summary

Clause	When it filters	Works with aggregates?	Common Use Case
WHERE	Before grouping	No	Filter individual rows
HAVING	After grouping	Yes	Filter aggregated results
AND / OR	Combine conditions	—	Multiple filter conditions
IN	Multiple specific values	—	Filter by a list of values
BETWEEN	Range of values	—	Numeric or date ranges
LIKE	Partial text match	—	Search within text fields