Ionic equations show what happens to ions in a chemical reaction. They don't necessarily show redox reactions, and they ignore any ions that are present in the system but don't actually take part in the reaction. For example, the ionic equation for the reaction HCl + NaOH →  NaCl + H2O, is H+ + OH- →  H2O. We ignore the Na+ and Cl- ions because they are present on both sides of the equation. However, half equations show what happens to electrons in redox reactions. They show the individual oxidation and reduction processes, as well as the changes in oxidation states of the species involved. The reaction we looked at above doesn't have any half equations, because it isn't a redox reaction - there is no transfer of electrons and none of the species change oxidation states.