# Differences in reading an item using its primary key
<a name="SQLtoNoSQL.ReadData.SingleItem"></a>

One common access pattern for databases is to read a single item from a table. You have to specify the primary key of the item you want.

**Topics**
+ [Reading an item using its primary key with SQL](#SQLtoNoSQL.ReadData.SingleItem.SQL)
+ [Reading an item using its primary key in DynamoDB](#SQLtoNoSQL.ReadData.SingleItem.DynamoDB)

## Reading an item using its primary key with SQL
<a name="SQLtoNoSQL.ReadData.SingleItem.SQL"></a>

In SQL, you would use the `SELECT` statement to retrieve data from a table. You can request one or more columns in the result (or all of them, if you use the `*` operator). The `WHERE` clause determines which rows to return.

The following is a `SELECT` statement to retrieve a single row from the *Music* table. The `WHERE` clause specifies the primary key values.

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

You can modify this query to retrieve only a subset of the columns.

```
SELECT AlbumTitle, Year, Price
FROM Music
WHERE Artist='No One You Know' AND SongTitle = 'Call Me Today'
```

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

## Reading an item using its primary key in DynamoDB
<a name="SQLtoNoSQL.ReadData.SingleItem.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 read an item from a table.

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#### [ DynamoDB API ]

With the DynamoDB API, you use the `PutItem` operation to add an item to a table.

DynamoDB provides the `GetItem` operation for retrieving an item by its primary key. `GetItem` is highly efficient because it provides direct access to the physical location of the item. (For more information, see [Partitions and data distribution in DynamoDB](HowItWorks.Partitions.md).)

By default, `GetItem` returns the entire item with all of its attributes.

```
{
    TableName: "Music",
    Key: {
        "Artist": "No One You Know",
        "SongTitle": "Call Me Today"
    }
}
```

You can add a `ProjectionExpression` parameter to return only some of the attributes.

```
{
    TableName: "Music",
    Key: {
        "Artist": "No One You Know",
        "SongTitle": "Call Me Today"
    },
    "ProjectionExpression": "AlbumTitle, Year, Price"
}
```

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

The DynamoDB `GetItem` operation is very efficient. It uses the primary key values to determine the exact storage location of the item in question, and retrieves it directly from there. The SQL `SELECT` statement is similarly efficient, in the case of retrieving items by primary key values.

The SQL `SELECT` statement supports many kinds of queries and table scans. DynamoDB provides similar functionality with its `Query` and `Scan` operations, which are described in [Differences in querying a table](SQLtoNoSQL.ReadData.Query.md) and [Differences in scanning a table](SQLtoNoSQL.ReadData.Scan.md).

The SQL `SELECT` statement can perform table joins, allowing you to retrieve data from multiple tables at the same time. Joins are most effective where the database tables are normalized and the relationships among the tables are clear. However, if you join too many tables in one `SELECT` statement application performance can be affected. You can work around such issues by using database replication, materialized views, or query rewrites.

DynamoDB is a nonrelational database and doesn't support table joins. If you are migrating an existing application from a relational database to DynamoDB, you need to denormalize your data model to eliminate the need for joins.

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#### [ PartiQL for DynamoDB ]

With PartiQL, you use the `ExecuteStatement` operation to read an item from a table, using the PartiQL `Select` statement.

```
SELECT AlbumTitle, Year, Price
FROM Music
WHERE Artist='No One You Know' AND SongTitle = 'Call Me Today'
```

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

**Note**  
 The select PartiQL statement can also be used to Query or Scan a DynamoDB table

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

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