Question: What does this symbol in database mean ⊆?

for example for projection in relational algebra :

ΠLR

take a subset of the attributes of a relation R,
where L ⊆ R.

Answer: 
L ⊆ R means L is subset of or equal to R.
L ⊆ R means literally L is a subset of R. 

In order to understand it here is an example, we have two sets A and B:

```
A = {1, 4, 5}
B = {1, 2, 3, 4, 5}

```

where A is a subset of B, because all numbers from set A can be found in set B. Also, we can see it as a diagram:

[![enter image description here](https://i.stack.imgur.com/CNv2km.png)](https://i.stack.imgur.com/CNv2km.png)

Now we can write it using the math notation:

```
A ⊆ B

```

I recommend to have a look on these pages, just to solid your knowledge:

<https://en.wikipedia.org/wiki/Subset>

<https://www.mathsisfun.com/sets/symbols.html>