Logical Query Processing Phases

Following are the main query clauses specified in the order that you are supposed to type them (known as “keyed-in order”):

1. SELECT

2. FROM

3. WHERE

4. GROUP BY

5. HAVING

6. ORDER BY

But as mentioned, the logical query processing order, which is the conceptual interpretation order, is different. It starts with the FROM clause. Here is the logical query processing order of the six main query clauses:

1. FROM

2. WHERE

3. GROUP BY

4. HAVING

5. SELECT

6. ORDER BY

Each phase operates on one or more tables as inputs and returns a virtual table as output. The output table of one phase is considered the input to the next phase. This is in accord with operations on relations that yield a relation. Note that if an ORDER BY is specified, the result isn’t relational.