93 lines
3.0 KiB
Markdown
93 lines
3.0 KiB
Markdown
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# UPDATE
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The `UPDATE` statement modifies existing data within a database table. Utilize this for atomic data changes based on specified criteria, ensuring data accuracy in application workflows.
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Update syntax:
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```sql
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UPDATE users SET username='software shinobi' WHERE id=1;
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```
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## Syntax breakdown:
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* `UPDATE users`: Identifies the target table (`users`) for the modification.
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* `SET username='software shinobi'`: Specifies the column (`username`) to update and its new value.
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* `WHERE id=1`: Filters which rows are affected. Only the row where `id` equals `1` will be updated.
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**Critical Impact:** Omitting the `WHERE` clause updates *every* row in the table. This operation is destructive; confirm intent before executing an `UPDATE` without a filter.
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## Update Multiple Rows
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Consider a `users` table state prior to updates:
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| id | username | about |
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| :-- | :------------ | :---- |
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| 2 | shinobi | NULL |
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| 3 | java team six | NULL |
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| 4 | tony | NULL |
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| 5 | shinobi | NULL |
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| 6 | java team six | NULL |
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| 7 | tony | NULL |
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Update all rows in the `about` column:
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```sql
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UPDATE users SET about='404 bio not found';
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```
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This query affects all six rows:
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```
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Query OK, 6 rows affected (0 .02 sec)
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Rows matched: 6 Changed: 6 Warnings: 0
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```
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`users` table state after updating all rows:
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| id | username | about |
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| :-- | :------------ | :---------------- |
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| 2 | shinobi | 404 bio not found |
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| 3 | java team six | 404 bio not found |
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| 4 | tony | 404 bio not found |
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| 5 | shinobi | 404 bio not found |
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| 6 | java team six | 404 bio not found |
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| 7 | tony | 404 bio not found |
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## Update Specific Row
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To update only a specific row, use the `WHERE` clause. Update the `about` column for the user with `id` 2:
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```sql
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UPDATE users SET about ='Hello World :)' WHERE id=2;
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```
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This targets a single row:
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```
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Query OK, 1 row affected (0.01 sec)
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Rows matched: 1 Changed: 1 Warnings: 0
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```
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`users` table state after updating a single row:
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| id | username | about |
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| :-- | :------------ | :---------------- |
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| 2 | shinobi | Hello World :) |
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| 3 | java team six | 404 bio not found |
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| 4 | tony | 404 bio not found |
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| 5 | shinobi | 404 bio not found |
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| 6 | java team six | 404 bio not found |
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| 7 | tony | 404 bio not found |
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## Updating Records Using Another Table
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Modify records in one table based on data from another table. This is achieved by joining or listing the tables involved in the `UPDATE` statement and specifying the join condition in the `WHERE` clause.
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Update the `about ` field in the `users` table using corresponding data from the `prospect_users` table where usernames match:
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```sql
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update users, prospect_users
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set users.about = prospect_users.about
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where prospect_users.username = users.username;
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```
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