> ## Documentation Index
> Fetch the complete documentation index at: https://docs.e6data.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Understand MERGE matches

> Match existing and incoming customers, then update an existing customer and

TutorialApplies to: Unified engine deployments

Match existing and incoming customers, then update an existing customer and
insert a new one.

The target is the stored table being changed. The source is the incoming set of
rows. ON decides which source and target rows match. The WHEN clauses choose
what happens to those rows. An upsert combines an update for an existing key
with an insert for a new key.

## Before you start

You need a writable Iceberg catalog with namespace creation, table creation, and
MERGE support. These examples create format-v2 tables in a private namespace.
They use inline data and do not need an external sample dataset.

Choose an unused namespace. The examples use `learn_merge_01`. Replace it
everywhere if it already exists. In Workbench, select your writable
catalog before running any SQL. Keep that catalog selected throughout the
lesson. Opening an example does not execute it.

## Set up the namespace

Run this setup once:

```sql theme={"theme":{"light":"github-light","dark":"github-dark"}}
SET spark.sql.caseSensitive = true;
CREATE NAMESPACE learn_merge_01;
```

Each example creates a separate table. Run an example's setup once. To start it
again, remove only that example's table first, using the cleanup section.
Scripts execute as separate operations; they are not a multi-statement
transaction.

## 1. Update an existing customer and insert a new one

Start with these target rows:

| id | name | balance | version | is\_deleted |
| - | - | - | - | - |
| 1 | Ada | 100 | 1 | false |
| 2 | Ben | 200 | 2 | false |
| 3 | Cyd | 300 | 1 | false |

```sql theme={"theme":{"light":"github-light","dark":"github-dark"}}
CREATE TABLE learn_merge_01.first_upsert (id BIGINT, name STRING, balance BIGINT, version BIGINT, is_deleted BOOLEAN)
USING iceberg TBLPROPERTIES ('format-version' = '2');
INSERT INTO learn_merge_01.first_upsert VALUES (1, 'Ada', 100, 1, false), (2, 'Ben', 200, 2, false), (3, 'Cyd', 300, 1, false);
```

Apply the incoming source below:

```sql theme={"theme":{"light":"github-light","dark":"github-dark"}}
MERGE INTO learn_merge_01.first_upsert AS t
USING (SELECT 2 AS id, 'Ben' AS name, 250 AS balance, 3 AS version, false AS is_deleted UNION ALL SELECT 4, 'Dan', 400, 1, false) AS s
ON t.id = s.id
WHEN MATCHED THEN UPDATE SET name = s.name, balance = s.balance, version = s.version
WHEN NOT MATCHED THEN INSERT (id,name,balance,version,is_deleted) VALUES (s.id,s.name,s.balance,s.version,s.is_deleted);
```

Check all rows:

```sql theme={"theme":{"light":"github-light","dark":"github-dark"}}
SELECT id, name, balance, version, is_deleted FROM learn_merge_01.first_upsert;
```

Expect these exact rows, in any display order:

| id | name | balance | version | is\_deleted |
| - | - | - | - | - |
| 1 | Ada | 100 | 1 | false |
| 2 | Ben | 250 | 3 | false |
| 3 | Cyd | 300 | 1 | false |
| 4 | Dan | 400 | 1 | false |

## Change one thing

Start the relevant example again from its original target rows. Drop only that
example table, rerun its CREATE TABLE and INSERT statements, then apply the
changed MERGE. Keep the lesson namespace.

Change customer 4 to customer 5 in the source. The final table should contain
keys 1, 2, 3, and 5. Customer 2 should still have balance 250.

## Check the result

Compare every row and field with the expected tables, including unchanged
customers and the number of copies of each key. A Succeeded status alone is not
the result check. Wait for confirmed publication before reading the target.

## Clean up

Remove only the private tables created by this lesson:

```sql theme={"theme":{"light":"github-light","dark":"github-dark"}}
DROP TABLE IF EXISTS learn_merge_01.first_upsert;
DROP NAMESPACE learn_merge_01;
```

## Continue

[Choose which rows MERGE updates](https://docs.e6data.com/transformation/learn/merge-conditions.md) or review the
[MERGE SQL reference](https://docs.e6data.com/transformation/sql/writes.md#merge).


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