# Using expressions in DynamoDB
<a name="Expressions"></a>

In Amazon DynamoDB, you can use *expressions* to specify which attributes to read from an item, write data when a condition is met, specify how to update an item, define queries and filter the results of a query.

This table describes the basic expression grammar and the available kinds of expressions.


| Expression type | Description | 
| --- | --- | 
| Projection expression | A projection expression identifies the attributes that you want to retrieve from an item when you use operations such as GetItem, Query, or Scan. | 
| Condition expression | A condition expression determines which items should be modified when you use the PutItem, UpdateItem, and DeleteItem operations. | 
| Update expression | An update expression specifies how UpdateItem will modify the attributes of an item— for example, setting a scalar value or removing elements from a list or a map. | 
| Key condition expression | A key condition expression determines which items a query will read from a table or index. | 
| Filter expression | A filter expression determines which items among the Query results should be returned to you. All the other results are discarded. | 

For information about expression syntax and more detailed information about each type of expression, see the following sections.

**Topics**
+ [Referring to item attributes when using expressions in DynamoDB](Expressions.Attributes.md)
+ [Expression attribute names (aliases) in DynamoDB](Expressions.ExpressionAttributeNames.md)
+ [Using expression attribute values in DynamoDB](Expressions.ExpressionAttributeValues.md)
+ [Using projection expressions in DynamoDB](Expressions.ProjectionExpressions.md)
+ [Using update expressions in DynamoDB](Expressions.UpdateExpressions.md)
+ [Condition and filter expressions, operators, and functions in DynamoDB](Expressions.OperatorsAndFunctions.md)
+ [DynamoDB condition expression CLI example](Expressions.ConditionExpressions.md)

**Note**  
For backward compatibility, DynamoDB also supports conditional parameters that do not use expressions. For more information, see [Legacy DynamoDB conditional parameters](LegacyConditionalParameters.md).  
New applications should use expressions rather than the legacy parameters.