"There can only be one row for each primary key, and it is very efficient to select on the primary because it is also an index. Unlike a unique index, a primary key cannot be NULL. Primary keys can also be used for things like foreign keys, constraints, sharing and more, and it is always important to define one, if possible."
edited to add second amendment, one of the create table examples was missing the closing semi colon.
spelling error in documentation
"There can only
beone row for each primary key, and it is very efficient to select on the primary because it is also an index. Unlike a unique index, a primary key cannot be NULL. Primary keys can also be used for things like foreign keys, constraints, sharing and more, and it is always important to define one, if possible."edited to add second amendment, one of the create table examples was missing the closing semi colon.