Hello Martin, On 8/22/2012 8:30 AM, Martin Gainty wrote:
assign realistic alias names OuterJoin should be called OuterJoin InnerJoin should be called InnerJoin
Almost! MySQL does not have a simple OUTER JOIN command (some RDBMSes call this a FULL OUTER JOIN). What we do have is the option to include the OUTER keyword into our LEFT or RIGHT joins. For example, both of these are acceptable:
LEFT OUTER JOIN LEFT JOIN Also, you need a space between "inner" and "join" as in INNER JOIN.
If you want only the most restricitive criteria that match resultsets from both select statements use INNER JOIN if you want all results from both resultsets (cartesian JOIN) use OUTER JOIN
Again, MySQL does not have a plain OUTER JOIN. If you want a full Cartesian product of two tables, use the 'comma join' syntax with no criteria for matching the tables in the WHERE clause.
SELECT ... FROM tableA, tableB WHERE .... Syntax details are located here: http://dev.mysql.com/doc/refman/5.5/en/join.html
Finally: Be aware FUNCTIONS such as AVG cast off indexing and should be avoided unless the FUNCTION(columnName) itself is indexed GROUP BY re-arranges your query so its best to introduce GROUP BY in stages
Not exactly. If you wrap a column in a function and attempt to use the results of that function in the WHERE clause, then you are correct. However based on the way your define your indexes, the data you process in a function may actually come from the index and save you a trip to the underlying table. In this case, the index could make your function faster by skipping an additional retrieval step.
use realistic alias names like Dept and EmployeeNumber and avoid aliases that cause confusion like 'a' or 'foo'
Excellent advice.
Develop in stages and write down what YOU EXPECT vs WHAT each query produces .. If the Individual Select doesnt produce expected results..STOP and correct the SELECT Statement
Also excellent advice.
Diagram out what you expect results from any of the INNER JOIN, OUTER JOIN, LEFT JOIN or RIGHT JOIN operations will produce If the executed JOIN Statement does not produce expected results STOP and correct the JOIN clause BEFORE incorporating more functionality Obfuscation and confusion can hopelessly sidetrack any intelligent analysis
Well put! -- Shawn Green MySQL Principal Technical Support Engineer Oracle USA, Inc. - Hardware and Software, Engineered to Work Together. Office: Blountville, TN -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe: http://lists.mysql.com/mysql
