# Differences in querying a table
<a name="SQLtoNoSQL.ReadData.Query"></a>

Another common access pattern is reading multiple items from a table, based on your query criteria.

**Topics**
+ [Querying a table with SQL](#SQLtoNoSQL.ReadData.Query.SQL)
+ [Querying a table in DynamoDB](#SQLtoNoSQL.ReadData.Query.DynamoDB)

## Querying a table with SQL
<a name="SQLtoNoSQL.ReadData.Query.SQL"></a>

When using SQL the `SELECT` statement lets you query on key columns, non-key columns, or any combination. The `WHERE` clause determines which rows are returned, as shown in the following examples.

```
/* Return a single song, by primary key */

SELECT * FROM Music
WHERE Artist='No One You Know' AND SongTitle = 'Call Me Today';
```

```
/* Return all of the songs by an artist */

SELECT * FROM Music
WHERE Artist='No One You Know';
```

```
/* Return all of the songs by an artist, matching first part of title */

SELECT * FROM Music
WHERE Artist='No One You Know' AND SongTitle LIKE 'Call%';
```

```
/* Return all of the songs by an artist, only if the price is less than 1.00 */

SELECT * FROM Music
WHERE Artist='No One You Know'
AND Price < 1.00;
```

Note that the primary key for this table consists of *Artist* and *SongTitle*.

## Querying a table in DynamoDB
<a name="SQLtoNoSQL.ReadData.Query.DynamoDB"></a>

In Amazon DynamoDB, you can use either the DynamoDB API or [PartiQL](https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/ql-reference.html) (a SQL-compatible query language) to query an item from a table.

------
#### [ DynamoDB API ]

With Amazon DynamoDB, you can use the `Query` operation to retrieve data in a similar fashion. The `Query` operation provides quick, efficient access to the physical locations where the data is stored. For more information, see [Partitions and data distribution in DynamoDB](HowItWorks.Partitions.md).

You can use `Query` with any table or secondary index. You must specify an equality condition for the partition key's value, and you can optionally provide another condition for the sort key attribute if it is defined.

The `KeyConditionExpression` parameter specifies the key values that you want to query. You can use an optional `FilterExpression` to remove certain items from the results before they are returned to you.

In DynamoDB, you must use `ExpressionAttributeValues` as placeholders in expression parameters (such as `KeyConditionExpression` and `FilterExpression`). This is analogous to the use of *bind variables* in relational databases, where you substitute the actual values into the `SELECT` statement at runtime.

Note that the primary key for this table consists of *Artist* and *SongTitle*.

The following are some DynamoDB `Query` examples.

```
// Return a single song, by primary key

{
    TableName: "Music",
    KeyConditionExpression: "Artist = :a and SongTitle = :t",
    ExpressionAttributeValues: {
        ":a": "No One You Know",
        ":t": "Call Me Today"
    }
}
```

```
// Return all of the songs by an artist

{
    TableName: "Music",
    KeyConditionExpression: "Artist = :a",
    ExpressionAttributeValues: {
        ":a": "No One You Know"
    }
}
```

```
// Return all of the songs by an artist, matching first part of title

{
    TableName: "Music",
    KeyConditionExpression: "Artist = :a and begins_with(SongTitle, :t)",
    ExpressionAttributeValues: {
        ":a": "No One You Know",
        ":t": "Call"
    }
}
```

```
// Return all of the songs by an artist, only if the price is less than 1.00

{
    TableName: "Music",
    KeyConditionExpression: "Artist = :a",
    FilterExpression: "Price < :p",
    ExpressionAttributeValues: {
        ":a": "No One You Know",
        ":p": 1.00
    }
}
```

**Note**  
A `FilterExpression` is applied after the `Query` reads matching items, so it does not reduce the read capacity consumed. Where possible, model your data so that range conditions use `KeyConditionExpression` on the sort key for efficient queries. For more information, see [Querying tables in DynamoDB](Query.md).

------
#### [ PartiQL for DynamoDB ]

With PartiQL, you can perform a query by using the `ExecuteStatement` operation and the `Select` statement on the partition key.

```
SELECT AlbumTitle, Year, Price
FROM Music
WHERE Artist='No One You Know'
```

Using the `SELECT` statement in this way returns all the songs associated with this particular `Artist`.

For code examples using `Select` and `ExecuteStatement`, see [PartiQL select statements for DynamoDB](ql-reference.select.md).

------