By Donald K. Burleson, Joe Celko, John Paul Cook, Peter Gulutzan
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Extra resources for Advanced SQL Database Programmers Handbook
The search conditions are logical predicates -- things that return TRUE, FALSE or UNKNOWN. But wait a minute, most programming languages work with Boolean logic and have only TRUE and FALSE logical values. SQL and Codd's first relational model have a thing called a NULL and it makes things ... interesting. The Null of It All A NULL is not a value; it is a marker for a value that is missing. SQL does not know why the value is missing -- semantics is your job. But SQL does have syntax to handle NULLs.
The dividend table must match exactly to the values of the divisor without any extra values. plane) = (SELECT COUNT(plane) FROM Hangar); This says that a pilot must have the same number of certificates as there planes in the hangar and these certificates all match to a plane in the hangar, not something else. The "something else" is shown by a created NULL from the LEFT OUTER JOIN. plane) because it does not work; it will tell you that the hangar has (n) planes in it and the pilot is certified for (n) planes, but not that those two sets of planes are equal to each other.
Com/oracle SQL NULL CHAPTER 5 Selection Introduction Continuing the look at basic relational operators and SQL, we get to an operation with an unfortunate name: Selection. Selection removes rows from a table which do not pass a search condition. It is the counterpart of Projection, which removes columns from tables. The reason the name is unfortunate is that SQL uses the keyword, SELECT, for the clause in a query that matches to the Projection operator and the keyword where for the clause in a query that matches to the selection operator.