Using boolean value in QGIS style rule

When using a boolean, you need to use the IS operator instead of LIKE. So your expression should be:

"field" is true

However, since you want to select the fields which are TRUE, you can also shorthen this and use this expression:

"field"

This works because in QGIS the fields will be selected, where the used expression returns TRUE. So on a booleanfield it is enough when the content is TRUE and therefore returning TRUE when evaluating it.

Proof:

Finnally I have found the solution:

"field" like 't'

Thank you

I would recommend, in order to see how QGIS views the values, to use the "all unique" button.

You should get a list of values.