Update Data_Cleaning.md

Case_Study_2_Pizza_Runner/Data_Cleaning.md

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+# Data Cleaning
 
+## Customer Orders Table
+
+In the `customer_orders` table, the `exclusions` and `extras` columns contain both empty strings and the word "null" stored as a string. Our goal is to replace these with actual `NULL` values. To achieve this, we use the `CASE WHEN` structure.
+
+```sql
+SELECT order_id,
+       customer_id,
+       pizza_id,
+       CASE WHEN exclusions = 'null' OR exclusions = '' THEN NULL ELSE exclusions END AS exclusions,
+       CASE WHEN extras = 'null' OR extras = '' THEN NULL ELSE extras END AS extras,
+       order_time
+FROM customer_orders;
+``` 
+output 
+
+![image](https://github.com/user-attachments/assets/c5775798-a74d-4bc9-85c7-bf79f6b170f9)
+
+Once I verified that the output matches the desired table structure, I proceed to update the table using the `UPDATE` and `SET` commands.
+
+```sql
+UPDATE customer_orders
+SET exclusions = CASE
+    WHEN exclusions = 'null' OR exclusions = '' THEN NULL ELSE exclusions
+END; 
+
+UPDATE customer_orders
+SET extras = CASE
+    WHEN extras = 'null' OR extras = '' THEN NULL ELSE extras
+END;
+```
+
+## Runner Orders Table
+
+When inspecting the `runner_orders` table, I noticed that the `pickup_time` column contains the word "null" stored as a string. The `distance` column also has "null" strings and includes the "km" suffix, despite it being a numeric field. We need to remove the "km" part and convert the column to a numeric data type. Lastly, the `cancellation` column also has string "null" values that need to be converted to `NULL`.
+
+```sql
+UPDATE runner_orders
+SET pickup_time = CASE
+    WHEN pickup_time = 'null' THEN NULL ELSE pickup_time
+END,
+    distance = CASE
+    WHEN distance = 'null' THEN NULL
+    WHEN distance LIKE '%km' THEN TRIM(distance, 'km') ELSE distance
+END,
+    duration = CASE
+    WHEN duration LIKE 'null' THEN NULL
+    WHEN duration LIKE '%min%' THEN TRIM(duration, 'minutes')
+	 ELSE duration
+END,
+    cancellation = CASE
+    WHEN cancellation = 'null' OR cancellation = '' THEN NULL ELSE cancellation
+END;
+```
+let's check everything is ok
+```sql
+SELECT * from runner_orders
+```
+output
+
+![image](https://github.com/user-attachments/assets/e2c11021-626a-4782-9fd5-c66d3e39dd63)
+
+### Changing Data Types
+
+To allow for time-based operations, I converted the `pickup_time` column to a `timestamp` format.
+
+```sql
+ALTER TABLE runner_orders
+ALTER COLUMN pickup_time
+TYPE timestamp 
+USING to_timestamp(pickup_time, 'YY/MM/DD HH24:MI');
+```
+
+Next, I converted the `distance` and `duration` columns to numeric values to enable mathematical operations.
+
+```sql
+ALTER TABLE runner_orders
+ALTER COLUMN distance
+TYPE float  -- You can also use numeric or real here
+USING distance::double precision;
+```
+
+```sql
+ALTER TABLE runner_orders
+ALTER COLUMN duration
+TYPE int  
+USING duration::integer;
+```