Querying historical data in SCADA systems - Mini Project: Build & Apply

Querying Historical Data

📖 Scenario: You work with a SCADA system that collects sensor data over time. You want to query the historical data to find specific readings.

🎯 Goal: Build a simple query to extract temperature readings from the historical data for analysis.

📋 What You'll Learn

Create a dictionary called historical_data with exact timestamp keys and temperature values

Add a variable called threshold to filter temperatures above a certain value

Use a dictionary comprehension called filtered_data to select entries with temperature above threshold

Print the filtered_data dictionary

💡 Why This Matters

🌍 Real World

SCADA systems collect sensor data over time. Querying historical data helps operators analyze trends and detect issues.

💼 Career

Knowing how to filter and query historical sensor data is important for roles in industrial automation, monitoring, and data analysis.

1

Create historical data dictionary

Create a dictionary called historical_data with these exact entries: '2024-06-01 08:00': 22.5, '2024-06-01 09:00': 23.0, '2024-06-01 10:00': 24.1, '2024-06-01 11:00': 25.3, '2024-06-01 12:00': 26.7

SCADA systems

# Create the historical_data dictionary with exact timestamps and temperatures
# Your code here

Hint

Use curly braces to create a dictionary with string keys and float values.

2

Add temperature threshold

Add a variable called threshold and set it to 24.0 to filter temperatures above this value.

SCADA systems

historical_data = {
    '2024-06-01 08:00': 22.5,
    '2024-06-01 09:00': 23.0,
    '2024-06-01 10:00': 24.1,
    '2024-06-01 11:00': 25.3,
    '2024-06-01 12:00': 26.7
}
# Add the threshold variable below
# Your code here

Hint

Just assign the number 24.0 to the variable named threshold.

3

Filter data above threshold

Use a dictionary comprehension called filtered_data to select entries from historical_data where the temperature is greater than threshold.

SCADA systems

historical_data = {
    '2024-06-01 08:00': 22.5,
    '2024-06-01 09:00': 23.0,
    '2024-06-01 10:00': 24.1,
    '2024-06-01 11:00': 25.3,
    '2024-06-01 12:00': 26.7
}
threshold = 24.0
# Create filtered_data dictionary using dictionary comprehension
# Your code here

Hint

Use dictionary comprehension syntax: {key: value for key, value in dict.items() if condition}.

4

Print filtered data

Print the filtered_data dictionary to display the filtered temperature readings.

SCADA systems

historical_data = {
    '2024-06-01 08:00': 22.5,
    '2024-06-01 09:00': 23.0,
    '2024-06-01 10:00': 24.1,
    '2024-06-01 11:00': 25.3,
    '2024-06-01 12:00': 26.7
}
threshold = 24.0
filtered_data = {time: temp for time, temp in historical_data.items() if temp > threshold}
# Print the filtered_data dictionary
# Your code here

Hint

Use print(filtered_data) to show the filtered dictionary.

Practice

1. What is the main purpose of querying historical data in SCADA systems?

easy

A. To control real-time device operations

B. To review past system behavior and analyze trends

C. To update firmware on sensors

D. To configure network settings

Solution

1. Step 1: Understand the role of historical data

   Historical data stores past readings and events from the system.

2. Step 2: Identify the purpose of querying it

   Querying helps analyze past behavior and detect trends or issues.

3. Final Answer:

   To review past system behavior and analyze trends -> Option B

4. Quick Check:

   Historical data = review past behavior [OK]

Hint: Historical data is for past info, not real-time control [OK]

Common Mistakes:

• Confusing historical data with real-time control
• Thinking it updates devices
• Assuming it changes network settings

2. Which SQL query correctly selects temperature readings from sensor 'S1' recorded after '2024-01-01'?

easy

A. SELECT sensor_id, timestamp FROM readings WHERE type = 'temperature' OR sensor_id = 'S1' AND timestamp > '2024-01-01';

B. SELECT * FROM readings WHERE sensor_id = 'S1' AND timestamp < '2024-01-01' AND type = 'temperature';

C. SELECT * FROM readings WHERE sensor_id == 'S1' AND timestamp > '2024-01-01' AND type = 'temperature';

D. SELECT * FROM readings WHERE sensor_id = 'S1' AND timestamp > '2024-01-01' AND type = 'temperature';

Solution

1. Step 1: Check correct SQL syntax for conditions

   Use single equals (=) for comparison and AND to combine conditions.

2. Step 2: Verify logical conditions match requirements

   SELECT * FROM readings WHERE sensor_id = 'S1' AND timestamp > '2024-01-01' AND type = 'temperature'; correctly filters sensor_id = 'S1', timestamp > '2024-01-01', and type = 'temperature'.

3. Final Answer:

   SELECT * FROM readings WHERE sensor_id = 'S1' AND timestamp > '2024-01-01' AND type = 'temperature'; -> Option D

4. Quick Check:

   Correct syntax and filters = SELECT * FROM readings WHERE sensor_id = 'S1' AND timestamp > '2024-01-01' AND type = 'temperature'; [OK]

Hint: Use = for comparison and AND to combine filters [OK]

Common Mistakes:

• Using '==' instead of '=' in SQL
• Mixing AND and OR without parentheses
• Using wrong comparison operators
• Filtering with wrong timestamp direction

3. Given this query:
SELECT timestamp, value FROM readings WHERE sensor_id = 'S2' AND timestamp BETWEEN '2024-03-01' AND '2024-03-05' ORDER BY timestamp DESC LIMIT 3;
What will be the output?

medium

A. The 3 latest readings from sensor S2 between March 1 and 5, sorted descending

B. An error because LIMIT cannot be used with ORDER BY

C. All readings from sensor S2 between March 1 and 5, unsorted

D. The 3 earliest readings from sensor S2 between March 1 and 5, sorted ascending

Solution

1. Step 1: Understand the WHERE and BETWEEN clause

   Filters readings from sensor 'S2' between '2024-03-01' and '2024-03-05'.

2. Step 2: Analyze ORDER BY and LIMIT

   ORDER BY timestamp DESC sorts newest first; LIMIT 3 returns top 3 newest readings.

3. Final Answer:

   The 3 latest readings from sensor S2 between March 1 and 5, sorted descending -> Option A

4. Quick Check:

   ORDER BY DESC + LIMIT 3 = latest 3 readings [OK]

Hint: ORDER BY DESC + LIMIT gets newest records first [OK]

Common Mistakes:

• Confusing ascending vs descending order
• Thinking LIMIT returns earliest records
• Assuming LIMIT causes syntax error with ORDER BY

4. You wrote this query to get pressure data:
SELECT * FROM readings WHERE sensor_id = 'P1' AND timestamp > '2024-02-01' AND type = 'pressure'
But it returns no results, even though data exists. What is the likely problem?

medium

A. The query is missing a GROUP BY clause

B. The sensor_id should be numeric, not string

C. The timestamp format is incorrect and does not match stored data

D. The type column does not exist in the readings table

Solution

1. Step 1: Check timestamp format correctness

   Timestamp format must match stored data format exactly to filter correctly.

2. Step 2: Verify other query parts

   Sensor_id as string is valid; GROUP BY not needed; type column likely exists.

3. Final Answer:

   The timestamp format is incorrect and does not match stored data -> Option C

4. Quick Check:

   Timestamp format mismatch = no results [OK]

Hint: Match timestamp format exactly to stored data [OK]

Common Mistakes:

• Assuming sensor_id must be numeric
• Adding unnecessary GROUP BY
• Ignoring timestamp format differences

5. You want to find the average temperature for each of sensors 'T1' and 'T2' during January 2024, but only for readings above 20°C. Which SQL query achieves this?

hard

A. SELECT sensor_id, AVG(value) FROM readings WHERE (sensor_id = 'T1' OR sensor_id = 'T2') AND timestamp BETWEEN '2024-01-01' AND '2024-01-31' AND value > 20 AND type = 'temperature' GROUP BY sensor_id;

B. SELECT AVG(value) FROM readings WHERE sensor_id IN ('T1', 'T2') AND timestamp >= '2024-01-01' AND timestamp <= '2024-01-31' AND value > 20 AND type = 'temperature';

C. SELECT sensor_id, AVG(value) FROM readings WHERE sensor_id = 'T1' AND sensor_id = 'T2' AND timestamp BETWEEN '2024-01-01' AND '2024-01-31' AND value > 20 AND type = 'temperature' GROUP BY sensor_id;

D. SELECT sensor_id, AVG(value) FROM readings WHERE sensor_id = 'T1' OR sensor_id = 'T2' AND timestamp BETWEEN '2024-01-01' AND '2024-01-31' AND value > 20 AND type = 'temperature';

Solution

1. Step 1: Filter sensors correctly

   Use (sensor_id = 'T1' OR sensor_id = 'T2') or sensor_id IN ('T1', 'T2') to include both sensors.

2. Step 2: Apply date and value filters with grouping

   Filter timestamp between January 1 and 31, value > 20, and type = 'temperature'. Group by sensor_id to get averages per sensor.

3. Step 3: Check query correctness

   SELECT sensor_id, AVG(value) FROM readings WHERE (sensor_id = 'T1' OR sensor_id = 'T2') AND timestamp BETWEEN '2024-01-01' AND '2024-01-31' AND value > 20 AND type = 'temperature' GROUP BY sensor_id; uses correct syntax with parentheses and GROUP BY; SELECT AVG(value) FROM readings WHERE sensor_id IN ('T1', 'T2') AND timestamp >= '2024-01-01' AND timestamp <= '2024-01-31' AND value > 20 AND type = 'temperature'; misses GROUP BY; SELECT sensor_id, AVG(value) FROM readings WHERE sensor_id = 'T1' AND sensor_id = 'T2' AND timestamp BETWEEN '2024-01-01' AND '2024-01-31' AND value > 20 AND type = 'temperature' GROUP BY sensor_id; has impossible condition; SELECT sensor_id, AVG(value) FROM readings WHERE sensor_id = 'T1' OR sensor_id = 'T2' AND timestamp BETWEEN '2024-01-01' AND '2024-01-31' AND value > 20 AND type = 'temperature'; lacks parentheses causing wrong logic.

4. Final Answer:

   SELECT sensor_id, AVG(value) FROM readings WHERE (sensor_id = 'T1' OR sensor_id = 'T2') AND timestamp BETWEEN '2024-01-01' AND '2024-01-31' AND value > 20 AND type = 'temperature' GROUP BY sensor_id; -> Option A

5. Quick Check:

   Correct filters + grouping = SELECT sensor_id, AVG(value) FROM readings WHERE (sensor_id = 'T1' OR sensor_id = 'T2') AND timestamp BETWEEN '2024-01-01' AND '2024-01-31' AND value > 20 AND type = 'temperature' GROUP BY sensor_id; [OK]

Hint: Use parentheses for OR and GROUP BY for averages per sensor [OK]

Common Mistakes:

• Missing GROUP BY when aggregating by sensor
• Using AND instead of OR between sensor_ids
• Incorrect timestamp filtering logic