

![0_image_0.png](0_image_0.png)

Example 2. **Classical examples of selection predicates include [14]:**

- minimum (or maximum) length: p(X, Y ) = (|X| > γ)
- **minimum (or maximum) frequency:**
p(X, Y ) = (|Y | > γ)
- **minimum (or maximum) area:**
p(X, Y ) = (|X|.|Y | > γ).

- requiring that an attribute (object) belongs (does not belong) to a concept: p(X, Y ) =
(A ∈ X).

- **. . .**

## 5.2 Projection Queries

For example, given gene expression data, a biologist might be interested in only a part of the genes. He may want to focus only on a subset of the genes, **for instance the genes**
A,B and C.

The most simple solution would be to extract the concepts not **on the whole dataset,**
but on a part of it containing only the columns A, B and C**, i.e., on a projection of**
the original database (see Fig. 2, right). If A **is a set of attributes, we denote** πA(Db) the projection of the database Db on the attributes of A.