Why is SELECT * considered harmful?

Question

Why is `SELECT *` bad practice? Wouldn't it mean less code to change if you added a new column you wanted? I understand that `SELECT COUNT(*)` is a performance problem on some DBs, but what if you really wanted every column?

Answer 1

If you actually want to view every column, then `SELECT *` is perfectly OK. It is usually discouraged for the times you do not to need to view every column. Generally, selecting fewer columns is less 'expensive' than many columns. Using `SELECT *` can often be a useful thing when you are attempting to get a sense of a table. As a warning, you will want to limit the number of rows you view when you do that though.