add example of drop column
add example of drop column

antlr/grammars-v4,antlr/grammars-v4,antlr/grammars-v4,antlr/grammars-v4,antlr/grammars-v4,antlr/grammars-v4,antlr/grammars-v4,antlr/grammars-v4,antlr/grammars-v4,antlr/grammars-v4,antlr/grammars-v4,antlr/grammars-v4