For an introduction to templating and template variables, refer to the [Templating][variables] and [Add and manage variables][add-template-variables] documentation.
For example, to create a variable that contains all values for tag `hostname`, specify a query like this in the query variable **Query**:
```sql
SHOW TAG VALUES WITH KEY = "hostname"
```
### Chain or nest variables
You can also create nested variables, sometimes called [chained variables]({{< relref "../../../dashboards/variables/add-template-variables#chained-variables" >}}).
For example, if you had a variable called `region`, you could have the `hosts` variable show only hosts from the selected region with a query like:
```sql
SHOW TAG VALUES WITH KEY = "hostname" WHERE region = '$region'
```
You can fetch key names for a given measurement:
```sql
SHOW TAG KEYS [FROM <measurement_name>]
```
If you have a variable with key names, you can use this variable in a group-by clause.
This helps you change group-by using the variable list at the top of the dashboard.
### Use ad hoc filters
InfluxDB supports the special **Ad hoc filters** variable type.
You can use this variable type to specify any number of key/value filters, and Grafana applies them automatically to all of your InfluxDB queries.
The InfluxDB data source supports two variable syntaxes for use in the **Query** field:
-`$<varname>`, which is easier to read and write but does not allow you to use a variable in the middle of a word.
```sql
SELECT mean("value") FROM "logins" WHERE "hostname" =~ /^$host$/ AND $timeFilter GROUP BY time($__interval), "hostname"
```
-`${varname}`
```sql
SELECT mean("value") FROM "logins" WHERE "hostname" =~ /^[[host]]$/ AND $timeFilter GROUP BY time($__interval), "hostname"
```
When you enable the **Multi-value** or **Include all value** options, Grafana converts the labels from plain text to a regex-compatible string, so you must use `=~` instead of `=`.
### Templated dashboard example
To view an example templated dashboard, refer to [InfluxDB Templated Dashboard](https://play.grafana.org/dashboard/db/influxdb-templated).
