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71 lines
1.9 KiB
HTML
<HTML>
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<TITLE>The POSTGRES95 User Manual - THE QUERY LANGUAGE</TITLE>
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</HEAD>
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<BODY>
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<font size=-1>
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<A HREF="pg95user.html">[ TOC ]</A>
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<H1>9. EXTENDING SQL: OPERATORS</H1>
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<HR>
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POSTGRES supports left unary, right unary and binary
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operators. Operators can be overloaded, or re-used
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with different numbers and types of arguments. If
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there is an ambiguous situation and the system cannot
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determine the correct operator to use, it will return
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an error and you may have to typecast the left and/or
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right operands to help it understand which operator you
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meant to use.
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To create an operator for adding two complex numbers
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can be done as follows. First we need to create a
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function to add the new types. Then, we can create the
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operator with the function.
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<pre>
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CREATE FUNCTION complex_add(complex, complex)
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RETURNS complex
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AS '$PWD/obj/complex.so'
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LANGUAGE 'c';
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CREATE OPERATOR + (
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leftarg = complex,
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rightarg = complex,
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procedure = complex_add,
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commutator = +
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);
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</pre>
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We've shown how to create a binary operator here. To
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create unary operators, just omit one of leftarg (for
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left unary) or rightarg (for right unary).
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If we give the system enough type information, it can
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automatically figure out which operators to use.
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<pre>
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SELECT (a + b) AS c FROM test_complex;
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+----------------+
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|c |
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+----------------+
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|(5.2,6.05) |
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+----------------+
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|(133.42,144.95) |
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+----------------+
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</pre>
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<HR>
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<font size=-1>
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<A HREF="pg95user.html">[ TOC ]</A>
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<A HREF="xaggr.html">[ Next ]</A>
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