---
title: "Deduplication"
weight: 16
type: docs
---
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# Deduplication
{{< label Batch >}} {{< label Streaming >}}

Deduplication removes rows that duplicate over a set of columns, keeping only the first one or the last one. In some cases, the upstream ETL jobs are not end-to-end exactly-once; this may result in duplicate records in the sink in case of failover. However, the duplicate records will affect the correctness of downstream analytical jobs - e.g. `SUM`, `COUNT` - so deduplication is needed before further analysis.

Flink uses `ROW_NUMBER()` to remove duplicates, just like the way of Top-N query. In theory, deduplication is a special case of Top-N in which the N is one and order by the processing time or event time.

The following shows the syntax of the Deduplication statement:

```sql
SELECT [column_list]
FROM table_name
QUALIFY ROW_NUMBER() OVER ([PARTITION BY col1[, col2...]] ORDER BY time_attr [asc|desc]) = 1
```

**Parameter Specification:**

- `ROW_NUMBER()`: Assigns an unique, sequential number to each row, starting with one.
- `PARTITION BY col1[, col2...]`: Specifies the partition columns, i.e. the deduplicate key.
- `ORDER BY time_attr [asc|desc]`: Specifies the ordering column, it must be a [time attribute]({{< ref "docs/concepts/sql-table-concepts/time_attributes" >}}). Currently Flink supports [processing time attribute]({{< ref "docs/concepts/sql-table-concepts/time_attributes" >}}#processing-time) and [event time attribute]({{< ref "docs/concepts/sql-table-concepts/time_attributes" >}}#event-time). Ordering by ASC means keeping the first row, ordering by DESC means keeping the last row.
- `WHERE rownum = 1`: The `rownum = 1` is required for Flink to recognize this query is deduplication.

{{< hint info >}}
Note: the above pattern must be followed exactly, otherwise the optimizer won’t be able to translate the query.
{{< /hint >}}

The following examples show how to specify SQL queries with Deduplication on streaming tables.

```sql
CREATE TABLE Orders (
  order_id  STRING,
  user        STRING,
  product     STRING,
  num         BIGINT,
  proctime AS PROCTIME()
) WITH (...);

-- remove duplicate rows on order_id and keep the first occurrence row,
-- because there shouldn't be two orders with the same order_id.
SELECT order_id, user, product, num
FROM (
  SELECT *,
    ROW_NUMBER() OVER (PARTITION BY order_id ORDER BY proctime ASC) AS row_num
  FROM Orders)
WHERE row_num = 1
```

{{< top >}}
