# How Can I 'Join' Two Metrics in a Prometheus Query?

Joining two metrics in Prometheus is commonly achieved through the use of the `*` operator or the `group` function, allowing you to perform operations between metrics based on their labels. Unlike traditional SQL databases, Prometheus does not have a direct `JOIN` function; instead, you leverage label matching and arithmetic operations to combine metric data. Here’s how to do it:

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### Step 1: Understand the Metrics

Let’s assume you have two metrics:

1. **`http_requests_total`**: Counts the total number of HTTP requests, with labels like `method`, `status`, and `instance`.
2. **`http_response_time_seconds`**: Measures the response time for HTTP requests, with similar labels.

### Step 2: Simple Metric Operation (Multiplication)

If you want to combine these two metrics through arithmetic operations, you can simply use the `*` operator. For instance, if you want to calculate the total request time for each request type, you can multiply the number of requests by their average response time:

```
sum(http_requests_total) * sum(http_response_time_seconds)
```

However, you may need to ensure both metrics are appropriately aligned. If they have different labels, you might want to use the `ignoring` or `on` keywords.

### Step 3: Using the `on` and `ignoring` Keywords

### 1. Using `on`

If both metrics share common labels (like `method`), you can join them based on those labels:

```
sum(http_requests_total) by (method)
  * on(method)
  sum(http_response_time_seconds) by (method)
```

This query sums up the total requests and the total response times for each HTTP method and then multiplies them based on the shared `method` label.

### 2. Using `ignoring`

If you want to join metrics while ignoring specific labels, you can use `ignoring`. For example, if you want to ignore the `status` label:

```
sum(http_requests_total)
  * ignoring(status)
  sum(http_response_time_seconds)

```

This will join the metrics without considering the `status` label, focusing only on the common labels that remain.

### Step 4: Example Query

Here’s an example that combines both metrics to calculate the average response time per request:

```
sum(rate(http_requests_total[5m])) by (method)
  / sum(rate(http_response_time_seconds[5m])) by (method)

```

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