It’s really easy to make incorrect assumptions about the data in a table:

stick figure with short curly hair, smiling: every hospital patient has a doctor right?

same person, three hours later, sad: why is everyone from May 2013 missing a doctor??

Some questions you might want to ask:

• Does this column have NULL or 0 or empty string values?

person, thinking: some patients have NULL names, that’s good to know

• How many different valves does this column have?

person, thinking: huh there are 3000 extra doctors in the system who never worked at the hospital, I should filter them out

• Are there duplicate values in this column?

person, thinking: sometimes a doctor has 2 appointments at the same time, that shouldn’t happen

• Does the id column in table A always have a match in table B?

person, thinking: why are there 213 doctor IDs with no match in the doctors table?!

A lot of these can also be enforced by NOT NULL or UNIQUE or FOREIGN KEY constraints on your tables.