Extracting Specific JSON Data with Device Filter in SQLite

Issue Overview: Difficulty Isolating JSON Field by Device in SQLite Queries

The user’s primary challenge revolves around extracting a specific JSON field (meter_reading) from a SQLite database table while filtering results to include only rows where the DEVICE column contains the value IP22. The table structure includes two columns: DEVICE (text) and DATA (text containing JSON objects).

Technical Context

1. Table Schema:

   CREATE TABLE "Energy" (
     "DEVICE" TEXT NOT NULL,
     "DATA" TEXT NOT NULL
   );
   

   • DEVICE stores identifiers like IP22 or IP15.
   • DATA stores JSON objects with fields such as meter_reading, timestamp, and others. Example JSON:

     {
       "cost": 0.6,
       "iso": "2022-08-31T08:03:28.322Z",
       "location": "Garage - Electricity",
       "meter_reading": 1.352,
       "timestamp": 1661933008,
       "total_cost": 0
     }
     

2. Initial Query:

   SELECT * FROM Energy 
   WHERE json_extract(Energy.DATA, '$.meter_reading') >= 0;
   

   • This returns all rows where meter_reading is non-negative, regardless of the DEVICE value. The user wanted to restrict results to IP22 devices only.

3. Root Problem:
   The query lacked a condition to filter rows by the DEVICE column. SQL’s WHERE clause requires explicit conditions for each filtering criterion. Without this, the query returns all rows satisfying the JSON condition, irrespective of the DEVICE value.

4. JSON Handling in SQLite:
   SQLite’s json_extract function retrieves values from JSON-formatted text. However, combining JSON operations with standard SQL filtering requires careful syntax to avoid logical errors.

Possible Causes: Why the Query Returned Unfiltered Results

1. Missing DEVICE Filter in the WHERE Clause

The original query did not include a condition to restrict results to DEVICE = 'IP22'. SQL processes WHERE clauses by evaluating all conditions; omitting the device filter meant the JSON condition alone determined the result set.

Example of Logical Flow:

Return all rows WHERE [meter_reading >= 0] is true.

No consideration for DEVICE values.

2. Misunderstanding SQL Operator Precedence and Logical AND

When combining multiple conditions, the AND operator must explicitly link them. A common mistake is assuming separate WHERE clauses or omitting AND, leading to unintended results.

Incorrect Approach:

SELECT * FROM Energy 
WHERE DEVICE = 'IP22', 
json_extract(Energy.DATA, '$.meter_reading') >= 0;  -- Syntax error

Correct Approach:

SELECT * FROM Energy 
WHERE DEVICE = 'IP22' 
AND json_extract(Energy.DATA, '$.meter_reading') >= 0;

3. Incorrect Use of Quotes for String Literals

SQL uses single quotes (') for string literals and double quotes (") for identifiers (e.g., table/column names). Using double quotes for IP22 would cause errors if IP22 were interpreted as an identifier.

Incorrect:

WHERE DEVICE = "IP22"  -- Treats IP22 as an identifier (column/table), not a value.

Correct:

WHERE DEVICE = 'IP22'  -- Treats IP22 as a string literal.

4. Data Type Mismatch in JSON Extraction

The meter_reading field in the JSON might be stored as text (e.g., "0.0") instead of a numeric type. This could cause unexpected behavior when comparing values (e.g., >= 0). SQLite’s json_extract returns JSON values in their native types, but if stored as text, explicit casting may be required.

Example Issue:

-- If meter_reading is stored as "0.0" (text):
json_extract(DATA, '$.meter_reading') >= 0  -- Returns false due to string comparison.

Troubleshooting Steps, Solutions & Fixes: Refining Queries for Accurate Results

Step 1: Validate Table Structure and Data

Before writing queries, inspect the table schema and sample data to ensure alignment with expectations.

Action:

1. Retrieve the table schema:

   .schema Energy
   

   Confirm DEVICE and DATA columns exist with the correct data types.

2. Sample data check:

   SELECT DEVICE, DATA FROM Energy LIMIT 2;
   

   Verify that DEVICE contains IP22/IP15 and DATA contains valid JSON.

Step 2: Write a Targeted Query with Combined Conditions

Combine the JSON extraction and device filter using AND.

Solution:

SELECT * 
FROM Energy 
WHERE DEVICE = 'IP22' 
AND json_extract(DATA, '$.meter_reading') >= 0;

Breakdown:

• DEVICE = 'IP22' filters rows to the specific device.
• json_extract(...) >= 0 ensures only non-negative meter readings are included.
• AND links the two conditions, requiring both to be true.

Step 3: Optimize JSON Extraction and Column Selection

To retrieve only the meter_reading and DEVICE, refine the SELECT clause.

Optimized Query:

SELECT 
  DEVICE, 
  json_extract(DATA, '$.meter_reading') AS meter_reading 
FROM Energy 
WHERE DEVICE = 'IP22' 
AND meter_reading >= 0;

Key Points:

• Use column aliases (AS meter_reading) for readability.
• SQLite allows referencing aliases in the WHERE clause if defined in SELECT.

Step 4: Handle Data Type Issues in JSON Fields

If meter_reading is stored as text or inconsistently typed, cast it explicitly.

Casting to Real:

SELECT 
  DEVICE, 
  CAST(json_extract(DATA, '$.meter_reading') AS REAL) AS meter_reading 
FROM Energy 
WHERE DEVICE = 'IP22' 
AND meter_reading >= 0;

Why This Works:

• CAST(... AS REAL) converts extracted JSON values to floating-point numbers.
• Avoids string-vs-number comparison errors (e.g., "0.0" vs. 0).

Step 5: Debugging Common Errors

Problem: Query Returns No Results Despite Valid Data

• Check Quotes: Ensure 'IP22' uses single quotes.
• Verify JSON Paths: Confirm $.meter_reading matches the JSON structure.
• Data Type Inspection: Use typeof() to debug JSON extracts:

  SELECT typeof(json_extract(DATA, '$.meter_reading')) FROM Energy;
  

Problem: Incorrect Filtering with Combined Conditions

• Test Conditions Separately:

  -- Check device filter alone:
  SELECT * FROM Energy WHERE DEVICE = 'IP22';
  
  -- Check JSON condition alone:
  SELECT * FROM Energy WHERE json_extract(DATA, '$.meter_reading') >= 0;
  

  Ensure both sub-queries return expected results before combining them.

Step 6: Advanced JSON Querying with json_tree

For nested JSON or arrays, use json_tree to flatten structures. However, this was misapplied in the user’s follow-up attempt.

Correct Usage:

SELECT 
  json_extract(value, '$.meter_reading') AS meter_reading 
FROM Energy, json_tree(Energy.DATA) 
WHERE DEVICE = 'IP22' 
AND json_tree.key = 'meter_reading';

Notes:

• json_tree generates a row for each JSON element. Filter with json_tree.key = 'meter_reading' to isolate specific fields.
• Avoid unnecessary joins if the JSON structure is flat.

Step 7: Integration with External Tools (e.g., Grafana)

To visualize data in Grafana:

1. Ensure Correct Data Types: Grafana may misinterpret text-based numbers. Use CAST in queries.
2. Time Series Format: Extract ISO timestamps as datetime fields:

   SELECT 
     json_extract(DATA, '$.iso') AS time,
     json_extract(DATA, '$.meter_reading') AS meter_reading 
   FROM Energy 
   WHERE DEVICE = 'IP22';
   

3. Grafana Data Source Configuration:
   • Set the time field as the timestamp in Grafana’s panel settings.
   • Use meter_reading as the metric.

Step 8: Schema Design Considerations for Future Projects

Storing JSON in SQLite offers flexibility but complicates querying. Consider normalizing data into separate columns for frequent queries:

Alternative Schema:

CREATE TABLE Energy (
  DEVICE TEXT NOT NULL,
  meter_reading REAL,
  timestamp INTEGER,
  location TEXT
);

Benefits:

• Simplified queries without JSON functions.
• Improved performance via indexing on DEVICE or timestamp.

This guide systematically addresses the core issue of filtering JSON data by device in SQLite, offering both immediate fixes and long-term best practices.