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POHow would this SQL query be evaluated by the query engine?
 

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  1. POHow would this SQL query be evaluated by the query engine?
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    <p>I was playing around with SQL Queries and this one came to my mind - <code>select x,y from table T</code>.</p> <p>Here x and y can be arithmetic expressions like 2.0/5.0 OR 1+2, the result has the number of rows of table T, with the first column having value 2.0/5.0 i.e. 0.444444 and the second column having value 3.</p> <p>I am curious to know as to how this query is executed. I have learned that in the SQL query engine there is a mapping from SQL representation of a query to an internal representation (like relational algebra) which is then optimized in order to yield an optimal query execution plan.</p> <p>Please correct my understanding if its wrong - For the query which I have stated, there would be an internal mapping from my query Q to an internal representation which is equivalent to that of <code>select * from table T</code>. Then for all the rows returned, n number of columns are projected out where n is the number of arithmetic expressions stated in the Q. And all rows for the n columns would have their respective evaluations of the arithmetic expressions.</p> <p>cheers</p>
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    1. USArnkrishn
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      1. POHow would this SQL query be evaluated by the query engine?
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      1. VTUpMod
    1. COminor quibble....2.0/5.0 is 0.4, not 0.4444444 :-)
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      1. USClyde
    2. COMany sql engines have a "explain" keyword
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      1. USlothar
 

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