It is important to understand that a joinis not a physical entityin other words, it does not exist in the actual database tables. A join is created by MySQL as needed, andit persists for the duration of the query execution. - maintainingreferential integrity 是说 MySQL 只允许合法的数据(fo...
http://www.w3schools.com/sql/sql_join.asp Scroll down to: W3 Schools said: Different SQL JOINs Before we continue with examples, we will list the types of the different SQL JOINs you can use: INNER JOIN: Returns all rows when there is at least one match in BOTH tables LEFT JOIN: ...