# Using expression attribute values in DynamoDB
<a name="Expressions.ExpressionAttributeValues"></a>

*Expression attribute values* in Amazon DynamoDB act as variables. They're substitutes for the actual values that you want to compare—values that you might not know until runtime. An expression attribute value must begin with a colon (`:`) and be followed by one or more alphanumeric characters.

For example, suppose that you wanted to return all of the `ProductCatalog` items that are available in `Black` and cost `500` or less. You could use a `Scan` operation with a filter expression, as in this AWS Command Line Interface (AWS CLI) example.

```
aws dynamodb scan \
    --table-name ProductCatalog \
    --filter-expression "contains(Color, :c) and Price <= :p" \
    --expression-attribute-values file://values.json
```

The arguments for `--expression-attribute-values` are stored in the `values.json` file.

```
{
    ":c": { "S": "Black" },
    ":p": { "N": "500" }
}
```

If you define an expression attribute value, you must use it consistently throughout the entire expression. Also, you can't omit the `:` symbol. 

Expression attribute values are used with key condition expressions, condition expressions, update expressions, and filter expressions.