definition of Expresion and operant?

Question

descripción of each and what's the differences

Answer 1

An expression is a combination of one or more values, operators, and SQL functions that evaluate to a value.

An operand is something on which you do an operation.
For example, in (2*3 + 4), 2*3 and 4 are the operands of +. In the case of SQL, when you do an UNION for example, the two SQL expressions from each side are the operands.

Answer 2

thanks¡¡ very good definitions, I now understand.