108 lines
2.9 KiB
Text
108 lines
2.9 KiB
Text
<Chapter Id="arrays">
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<Title>Arrays</Title>
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<Para>
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<Note>
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<Para>
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This must become a chapter on array behavior. Volunteers? - thomas 1998-01-12
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</Para>
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</Note>
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</Para>
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<Para>
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<ProductName>Postgres</ProductName> allows attributes of an instance to be defined
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as fixed-length or variable-length multi-dimensional
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arrays. Arrays of any base type or user-defined type
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can be created. To illustrate their use, we first create a
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class with arrays of base types.
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<ProgramListing>
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CREATE TABLE SAL_EMP (
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name text,
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pay_by_quarter int4[],
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schedule text[][]
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);
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</ProgramListing>
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</Para>
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<Para>
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The above query will create a class named SAL_EMP with
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a <FirstTerm>text</FirstTerm> string (name), a one-dimensional array of <FirstTerm>int4</FirstTerm>
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(pay_by_quarter), which represents the employee's
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salary by quarter and a two-dimensional array of <FirstTerm>text</FirstTerm>
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(schedule), which represents the employee's weekly
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schedule. Now we do some <FirstTerm>INSERTS</FirstTerm>s; note that when
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appending to an array, we enclose the values within
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braces and separate them by commas. If you know <FirstTerm>C</FirstTerm>,
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this is not unlike the syntax for initializing structures.
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<ProgramListing>
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INSERT INTO SAL_EMP
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VALUES ('Bill',
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'{10000, 10000, 10000, 10000}',
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'{{"meeting", "lunch"}, {}}');
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INSERT INTO SAL_EMP
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VALUES ('Carol',
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'{20000, 25000, 25000, 25000}',
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'{{"talk", "consult"}, {"meeting"}}');
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</ProgramListing>
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By default, <ProductName>Postgres</ProductName> uses the "one-based" numbering
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convention for arrays -- that is, an array of n elements starts with array[1] and ends with array[n].
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Now, we can run some queries on SAL_EMP. First, we
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show how to access a single element of an array at a
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time. This query retrieves the names of the employees
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whose pay changed in the second quarter:
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<ProgramListing>
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SELECT name
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FROM SAL_EMP
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WHERE SAL_EMP.pay_by_quarter[1] <>
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SAL_EMP.pay_by_quarter[2];
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+------+
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|name |
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+------+
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|Carol |
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+------+
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</ProgramListing>
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</Para>
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<Para>
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This query retrieves the third quarter pay of all
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employees:
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<ProgramListing>
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SELECT SAL_EMP.pay_by_quarter[3] FROM SAL_EMP;
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+---------------+
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|pay_by_quarter |
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+---------------+
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|10000 |
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+---------------+
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|25000 |
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+---------------+
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</ProgramListing>
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</Para>
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<Para>
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We can also access arbitrary slices of an array, or
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subarrays. This query retrieves the first item on
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Bill's schedule for the first two days of the week.
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<ProgramListing>
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SELECT SAL_EMP.schedule[1:2][1:1]
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FROM SAL_EMP
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WHERE SAL_EMP.name = 'Bill';
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+-------------------+
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|schedule |
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+-------------------+
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|{{"meeting"},{""}} |
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+-------------------+
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</ProgramListing>
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</Para>
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</Chapter>
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