How Can I Simulate Upserting in Microsoft Access Using SQL?

Simulate Upsert operation in Microsoft Access

Normally, inserting and updating rows into a table are two independent processes. But what if you want to perform both operations in one query? This operation is often called "Upsert".

In Microsoft Access 2000 and later, you can use an UPDATE query with a LEFT JOIN to simulate an Upsert operation. The syntax is as follows:

UPDATE b
LEFT JOIN a ON b.id = a.id
SET a.f1 = b.f1, a.f2 = b.f2, a.f3 = b.f3;

In this query:

• Table "b" is the new data you want to insert or update.
• Table "a" is an existing table you want to operate on.
• A left join ensures that any rows that do not exist in table "a" are automatically joined with NULL values.

So if a row with ID b.id exists in table "a", that row will be updated with the value from table "b". Otherwise, a new row will be inserted into table "a".

Example:

Consider the following form:

<code>| 表 "a" | 表 "b" |
|---|---|
| ID | Field1 | ID | Field1 |
| 1 | ValueA1 | 1 | ValueB1 |
| 2 | ValueA2 | 2 | ValueB2 |
| 3 | ValueA3 | 3 | ValueB3 |
|   |         | 4 | ValueB4 |</code>

Running the UPDATE query above will result in the following updated table "a":

<code>| ID | Field1 |
|---|---|
| 1 | ValueB1 |
| 2 | ValueB2 |
| 3 | ValueB3 |
| 4 | ValueB4 |  **(ID = 4 的新行已插入)**</code>

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