PostGIS provides two types of map-algebra functions, both of which return a new raster with one band. The type you use depends on the problem being solved and the number of raster bands involved.
The first map-algebra function (ST_MapAlgebra() or ST_MapAlgebraExpr()) depends on a valid, user-provided PostgreSQL algebraic expression that is called for every pixel. The expression can be as simple as an equation, or as complex as a logic-heavy SQL expression. If the map-algebra operation only requires at most two raster bands, and the expression is not complicated, you should have no problems using the expression-based map-algebra function.
The second map-algebra function (ST_MapAlgebra(), ST_MapAlgebraFct(), or ST_MapAlgebraFctNgb() ...
