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landing/docs/Introduction-To-Databases/009-update.md

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+# UPDATE
+
+As the name suggests, whenever you have to update some data in your database, you would use the `UPDATE` statement.
+
+You can use the `UPDATE` statement to update multiple columns in a single table.
+
+The syntax would look like this:
+
+```sql
+UPDATE users SET username='bobbyiliev' WHERE id=1;
+```
+
+Rundown of the statement:
+
+* `UPDATE users`: First, we specify the `UPDATE` keyword followed by the table that we want to update.
+* `username='bobbyiliev'`: Then we specify the columns that we want to update and the new value that we want to set.
+* `WHERE id=1`: Finally, by using the `WHERE` clause, we specify which user should be updated. In our case, it is the user with ID 1.
+
+The most important thing that you need to keep in mind is that if you don't specify a `WHERE` clause, all of the entries inside the `users` table would be updated, and all users would have the `username` set to `bobbyiliev`.
+
+> Important: You need to be careful when you use the `UPDATE` statement without a `WHERE` clause as every single row will be updated.
+
+If you have been following along all of the user entries in our `users` table, it currently have no data in the `about` column:
+
+```
++----+----------+-------+
+| id | username | about |
++----+----------+-------+
+|  2 | bobby    | NULL  |
+|  3 | devdojo  | NULL  |
+|  4 | tony     | NULL  |
+|  5 | bobby    | NULL  |
+|  6 | devdojo  | NULL  |
+|  7 | tony     | NULL  |
++----+----------+-------+
+```
+
+Let's go ahead and update this for all users and set the column value to `404 bio not found`, For example:
+
+```sql
+UPDATE users SET about='404 bio not found';
+```
+
+The output would let you know how many rows have been affected by the query:
+
+```
+Query OK, 6 rows affected (0.02 sec)
+Rows matched: 6  Changed: 6  Warnings: 0
+```
+
+Now, if you were to run a select for all `users`, you would get the following result:
+
+```
++----+----------+-------------------+
+| id | username | about             |
++----+----------+-------------------+
+|  2 | bobby    | 404 bio not found |
+|  3 | devdojo  | 404 bio not found |
+|  4 | tony     | 404 bio not found |
+|  5 | bobby    | 404 bio not found |
+|  6 | devdojo  | 404 bio not found |
+|  7 | tony     | 404 bio not found |
++----+----------+-------------------+
+```
+
+Let's now say that we wanted to update the `about` column for the user with an id of 2. In this case, we need to specify a `WHERE` clause followed by the ID of the user that we want to update as follows:
+
+```sql
+UPDATE users SET about='Hello World :)' WHERE id=2;
+```
+
+The output here should indicate that only 1 row was updated:
+
+```
+Query OK, 1 row affected (0.01 sec)
+Rows matched: 1  Changed: 1  Warnings: 0
+```
+
+Now, if you again run the `SELECT id, username, about FROM users` query, you would see that the user with `id` of 2 now has an updated `about` column data:
+
+```
++----+----------+-------------------+
+| id | username | about             |
++----+----------+-------------------+
+|  2 | bobby    | Hello World :)    |
+|  3 | devdojo  | 404 bio not found |
+|  4 | tony     | 404 bio not found |
+|  5 | bobby    | 404 bio not found |
+|  6 | devdojo  | 404 bio not found |
+|  7 | tony     | 404 bio not found |
++----+----------+-------------------+
+```
+
+## Updating records using another table
+
+As we've seen in the previous section, you can insert multiple rows in your table using another table. You can use the same principle for the update command.
+
+To do that you simply have to list all needed table in the `update` section, then you have to explain which action you want to perform on the table, and then you need to link the table together.
+
+For example, if you want to update the `about` field in the `users` table using the content of the `about` field in the `prospect_users` table, you would do something like this:
+
+```sql
+update users, prospect_users
+set users.about = prospect_users.about
+where prospect_users.username = users.username;
+```