Suggest an edit

Improve this article

Refine the answer for “What does the MERGE command do?”. Your changes go to moderation before they’re published.

Approval required

Content

What you’re changing

Title (EN)

Short answer (EN)

Shown above the full answer for quick recall.

Answer (EN)

The `MERGE` **command** in SQL is used to **combine data from two tables** - it lets you insert new records, update existing ones, and remove duplicates if needed, all at once. It's often called **"UPSERT"** (update + insert).

**Syntax (general form):**

```sql
MERGE INTO Target_Table AS T
USING Source AS S
ON T.Key = S.Key
WHEN MATCHED THEN
    UPDATE SET T.Field1 = S.Field1, T.Field2 = S.Field2
WHEN NOT MATCHED THEN
    INSERT (Field1, Field2) VALUES (S.Field1, S.Field2);
```

**Example:**

```sql
MERGE INTO Employees AS E
USING NewEmployees AS N
ON E.ID = N.ID
WHEN MATCHED THEN
    UPDATE SET E.Name = N.Name, E.DepartmentID = N.DepartmentID
WHEN NOT MATCHED THEN
    INSERT (ID, Name, DepartmentID) VALUES (N.ID, N.Name, N.DepartmentID);
```

### What it does

1. If a record with that `ID` already exists in the `Employees` table → it updates the data (`UPDATE`).
2. If the record doesn't exist → it adds a new one (`INSERT`).

**Why it's used:**

- It's convenient for syncing tables, importing new data while updating old data at the same time.
- It avoids the need for separate code to check whether a record exists before inserting or updating it.

> Put simply, `MERGE` is a command for **"update or insert" a record into a table, depending on whether it already exists**.

Markdown · drag & drop images · ⌘B / ⌘I shortcuts213 words

For the reviewer

Note to the moderator (optional)

Visible only to the moderator. Helps review go faster.