<p>What is the string concatenation operator in Oracle SQL?  </p>

<p>Are there any "interesting" features I should be careful of?  </p>

<p>(This seems obvious, but I couldn't find a previous question asking it).  </p>

It is
```
||
```
, for example:
```
```
select 'Mr ' || ename from emp;
```
```
The only "interesting" feature I can think of is that
```
'x' || null
```
returns
```
'x'
```
, not
```
null
```
as you might perhaps expect.