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Figure 5-7. Some rows from the left outer join (Listing 5-4)
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Who Coaches the Coaches, or Who Is My Grandmother
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Our self join shows us one level deep of members and coaches. If we look at the rows in Figure 5-7, we can see that Thomas Sexton is coached by William Cooper, who is in turn coached by Brenda Nolan, who doesn t have a coach. The hierarchy isn t all that interesting for this problem, but there are several analogous situations where the hierarchy is of considerable interest. Genealogy is one. Consider the data model and Person table in Figure 5-8. We record information (just a wee bit!) about each person, including who that person s mother is (we ll just consider birth mothers or it will get too complicated).
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Person
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Figure 5-8. Data model for people and their birth mothers
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CHAPTER 5 SE LF JOINS
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The relationship in Figure 5-8 can be read clockwise as a person can be the mother of several other people and in the other direction as a person has at most one mother and might have none. Now in real life, that last statement doesn t sound right surely everyone has a mother. However, our database is keeping only the data we know about, and unless we trace everyone back to the primeval slime, there will be some people in our table whose mother we do not know. Brenda is one. The table and model in Figure 5-8 are the same as our member and coach example, but a question like Who is Sue s grandmother seems a bit more likely than Who coaches my coach So how do we get information about people along with information about their mothers Just as in the previous section, we need to join the Person table to itself. (Don t forget to make the join an outer join so you don t lose Brenda.) The SQL is in Listing 5-5, and the Access diagrammatic interface for the join is shown in Figure 5-9, along with the resulting table. I ve given the first copy of the table the alias p for person and the second copy the alias m for mother.
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Listing 5-5. People and Their Mothers SELECT p.ID, p.FName, p.Mother, m.ID, m.FName, m.Mother FROM Person p LEFT OUTER JOIN Person m on p.Mother = m.ID
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Figure 5-9. Finding people and their mothers: Access diagram for the join (top) and the
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resulting table (bottom)
Now what about going back the next generation For that, we want to take the result table in Figure 5-9 and join that to another copy of the People table (with the alias g for grandmother). The SQL is in Listing 5-6, and the diagram and result table are in Figure 5-10.
C HA PTER 5 S ELF JOINS
Listing 5-6. People and Their Mothers and Maternal Grandmothers SELECT p.ID, p.FName, p.Mother, m.ID, m.FName, m.Mother, g.ID, g.FName, g.Mother FROM (Person p LEFT JOIN Person m ON p.Mother = m.ID) LEFT JOIN Person g ON m.Mother = g.ID
Figure 5-10. Finding three generations: Access diagram for the joins (top) and the resulting
table (bottom)
Clearly, we can keep making more and more self joins until we run out of generations. These sorts of hierarchical queries are likely to turn up whenever we have self relationships. One small catch is that we must specify the number of joins in each query. Standard SQL doesn t have the notion of a query that automatically keeps doing the self joins until it runs out of generations, such as Find all my female ancestors. 1
A Calculus Approach to Self Joins
The questions in the previous sections were all quite easy to answer once we realized we needed a self join. Sometimes, however, these realizations don t always come when you need them. Whenever my mind goes blank when faced with a query, I always resort to a calculus approach.
1. Some implementations of SQL do support recursive queries that can track through self relationships. Check your documentation for key phrases like WITH and CONNECT BY.
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