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><H1
><A
NAME="ARRAYS"
>Chapter 10. Arrays</A
></H1
><P
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>This must become a chapter on array behavior. Volunteers? - thomas 1998-01-12</P
></BLOCKQUOTE
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><P
> <SPAN
CLASS="PRODUCTNAME"
>Postgres</SPAN
> allows attributes of an instance to be defined
as fixed-length or variable-length multi-dimensional
arrays. Arrays of any base type or user-defined type
can be created. To illustrate their use, we first create a
class with arrays of base types.
<PRE
CLASS="PROGRAMLISTING"
>CREATE TABLE SAL_EMP (
name text,
pay_by_quarter int4[],
schedule text[][]
);</PRE
></P
><P
> The above query will create a class named SAL_EMP with
a <I
CLASS="FIRSTTERM"
>text</I
> string (name), a one-dimensional array of <I
CLASS="FIRSTTERM"
>int4</I
>
(pay_by_quarter), which represents the employee's
salary by quarter and a two-dimensional array of <I
CLASS="FIRSTTERM"
>text</I
>
(schedule), which represents the employee's weekly
schedule. Now we do some <I
CLASS="FIRSTTERM"
>INSERTS</I
>s; note that when
appending to an array, we enclose the values within
braces and separate them by commas. If you know <I
CLASS="FIRSTTERM"
>C</I
>,
this is not unlike the syntax for initializing structures.
<PRE
CLASS="PROGRAMLISTING"
>INSERT INTO SAL_EMP
VALUES ('Bill',
'{10000, 10000, 10000, 10000}',
'{{"meeting", "lunch"}, {}}');
INSERT INTO SAL_EMP
VALUES ('Carol',
'{20000, 25000, 25000, 25000}',
'{{"talk", "consult"}, {"meeting"}}');</PRE
>
By default, <SPAN
CLASS="PRODUCTNAME"
>Postgres</SPAN
> uses the "one-based" numbering
convention for arrays -- that is, an array of n elements starts with array[1] and ends with array[n].
Now, we can run some queries on SAL_EMP. First, we
show how to access a single element of an array at a
time. This query retrieves the names of the employees
whose pay changed in the second quarter:
<PRE
CLASS="PROGRAMLISTING"
>SELECT name
FROM SAL_EMP
WHERE SAL_EMP.pay_by_quarter[1] <>
SAL_EMP.pay_by_quarter[2];
+------+
|name |
+------+
|Carol |
+------+</PRE
></P
><P
> This query retrieves the third quarter pay of all
employees:
<PRE
CLASS="PROGRAMLISTING"
>SELECT SAL_EMP.pay_by_quarter[3] FROM SAL_EMP;
+---------------+
|pay_by_quarter |
+---------------+
|10000 |
+---------------+
|25000 |
+---------------+</PRE
></P
><P
> We can also access arbitrary slices of an array, or
subarrays. This query retrieves the first item on
Bill's schedule for the first two days of the week.
<PRE
CLASS="PROGRAMLISTING"
>SELECT SAL_EMP.schedule[1:2][1:1]
FROM SAL_EMP
WHERE SAL_EMP.name = 'Bill';
+-------------------+
|schedule |
+-------------------+
|{{"meeting"},{""}} |
+-------------------+</PRE
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