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POWhat happens if I select two tables with no WHERE clause?
 

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  1. POWhat happens if I select two tables with no WHERE clause?
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    <p>I had a technical interview last week, and my interviewer asked me what happens if I run the following query:</p> <p><code>SELECT * FROM tbl1, tbl2</code></p> <p>I think I answered it correctly, but it wasn't an in-depth answer. </p> <p>I said that I would select all the columns in both tables. For example if tbl1 has 3 columns, and tbl2 has 4 columns. The result set would have 7 columns. </p> <p>Then he asked me why 7? and I said because I was selecting everything from each table. </p> <p>That was a bad answer, but I couldn't think of anything else. </p> <p>To cut to the chase, after the interviewed I executed the latter statement using two tables. </p> <p>Table A, had 3 animal: dog, cat and elephant.</p> <p>Table B had 2 names: Mat and Beth</p> <p>This is the result set that I got after the statement being executed:</p> <pre><code>********************************************* | id_tbl1 | name_tbl1 | id_tbl2 | name_tbl2 | ********************************************* | 1 | dog | 1 | Mat | | 2 | cat | 1 | Mat | | 3 | elephant | 1 | Mat | | 1 | dog | 2 | Beth | | 2 | cat | 2 | Beth | | 3 | elephant | 2 | Beth | ********************************************* </code></pre> <p>So my question is, why does the statement behaves like that? </p> <p>In other words:</p> <p>Why does the Table B's records repeat themselves until I reach the end of table A, and then it starts all over again? </p> <p>How would you have answered the question in a way that it would've "WOW'd" the interviewer? </p> <p>If this question does not belong to SO, feel free to delete it or close it!</p>
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    1. USILikeTacos
    1. USILikeTacos
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      1. POWhat happens if I select two tables with no WHERE clause?
      1. USEjaz
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    1. COThey wee probably looking for the term "cross join" or "cartesian product". Also could have mentioned ANSI 89 vs ANSI 92 join syntax.
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      1. USMartin Smith
    2. COIt behaves like this because you mentioned no criteria for join. so it's like `ON 1=1` - which means every line matches every line. But the exact terms this is called are the ones @MartinSmith mentioned.
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      1. USponcha
    3. COas Martin Smith said, its producing the cross join ( product ) of both tables. So to say Y x X rows will be putout... or for your "WOW answer" you should have said: "for doing those, you shall be punished by a slow "explain query" death.
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      1. USNajzero
 

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