> For the complete documentation index, see [llms.txt](https://faris-shi84.gitbook.io/oracle-12c-notes/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://faris-shi84.gitbook.io/oracle-12c-notes/appendix-table-lock.md).

# Appendix: Table Lock

Table locks can be used to prevent users from mistakenly overwriting changes made by other users.

* `shared lock`: means other users can view the data in the table, but they can not perform any `DDL` and `DML` operations while the lock is active.
* `exclusive lock`: it is the most restrict lock that prevent users view and modify data util the lock is released.
* A lock is released automatically when the transaction control statement is issued, a `DDL` statement is executed, or exiting system by using `EXIT` command.
* `SELECT ...FOR UPDATE` command places a shared lock on the records to be changed and prevent other users from acquiring a lock on the same records.

```sql
LOCK TABLE tablename in SHARED MODE;

LOCK TABLE tablename in EXCLUSIVE MODE;

SELECT columnname,...
    FROM tablename
    [WHERE condition]
    FOR UPDATE;
```