Every table you have queried began with a CREATE TABLE statement that defined its structure: the columns, their data types, and the rules that govern valid data. Schema design is where data quality is won or lost, because the structure you declare determines what the database will accept, reject, and guarantee about every row from that point forward, long before any query runs.
Choosing the right data type for each column is foundational. Types like INTEGER, TEXT, NUMERIC, BOOLEAN, and TIMESTAMPTZ tell the engine how to store, compare, and validate values, and a well-chosen type both saves space and prevents whole classes of invalid data. A column declared as a date simply cannot hold the string "yesterday", which stops bad data at the door.
Constraints go further, encoding business rules directly into the schema. PRIMARY KEY, FOREIGN KEY, NOT NULL, UNIQUE, and CHECK turn assumptions into enforced guarantees, so the database itself refuses to store data that violates them. Designing types and constraints deliberately is what makes a database trustworthy, turning integrity from something applications must remember into something the engine enforces automatically.