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Checking Queries
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You ve written a query, run it, and got some results. Is all well and good Not necessarily. Just as first attempts at a query may not be elegant, neither might they be correct. Mistakes might arise from simple errors in the query syntax. These are usually easy to spot and correct. However, errors that result from subtle misunderstandings of the question or of the data can be more difficult to find. I can t offer a foolproof way of checking that your query is correct, but I can give you some ideas for catching potential errors. Basically, they boil down to checking that you do not have extra, incorrect rows in your result and checking that you aren t missing any rows. In this section, we will look at ways to notice that your query has a problem. In the next chapter, we will look at some of the common mistakes that might be behind errors.
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CH A PT ER 1 0 H O W TO A PPRO A CH A Q UERY
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Check a Row That Should Be Returned
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Take a look at your data and determine one record or row that should be returned by your query. For example, in our example about teams with managers as coaches, check through the tables and find at least one team that satisfies the query. In Figure 10-12, we see that TeamB satisfies the conditions, so check that this team is in your output. Remember that some queries may quite legitimately have no output. For example, it s perfectly reasonable that, with the data we have at any particular time, no teams are managed by a coach. However, your query must work in all situations. If it is at all possible, make a copy of the tables, alter the data so that a row meets the condition, and check that it is returned correctly.
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Check a Row That Should Not Be Returned
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Similar to checking for a row that should be returned, look through the data and find a team that doesn t have a coach as a manager. TeamA s manager (member 239) does not appear as a coach in the Member table, so make sure that team is not included in your output. Once again, it is a good idea to use some dummy data to check this if the real data does not cover all eventualities.
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Check Boundary Conditions
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If your query has any sort of numeric comparison, you should check it carefully. Consider a query where we want to find people who have been members of our club for more than 10 years. To be certain of the correctness, we need to check three possibilities: Make sure no record is returned for someone with less than 10 years of membership (for example, 8 years of membership). Make sure that someone who has belonged to the club for 12 years does get his record retrieved. Check for someone who has been a member for exactly 10 years. The last boundary condition is always tricky. It depends whether we use > or >= in the select condition. It might also mean reconsidering the original question. In this case, the request is probably for members with 10 or more years of membership. You should check with your users if there is any doubt. Finding data in your tables that falls exactly on the boundaries is not always easy. However, you can always change the numeric value in your query. Find a particular member and change the value you are checking against in the query to match their years of membership.
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CHAPTER 10 HOW TO APPROAC H A QUERY
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If Harry has been a member for 16 years, change the query to check for 16 or more years of membership, and see if Harry is included (or not) as you expect. Another important boundary condition, especially for aggregates and counts (covered in 8), is the value 0. Consider a query such as Find members who have entered fewer than six tournaments. Doing a grouped-by count on the Entry table will return some rows for sure, and we can check for those who have less than, more than, or exactly six entries. However, what about members who have never entered a tournament They won t appear in the Entry table at all and will be missing from the results. So whenever aggregates are involved, always check what happens for a count of 0. For example, does your query return members who have entered no tournaments
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