Non-Atomic Values

One of the tenets of the relational model is that the attributes of a relation are atomic. Postgres does not have this restriction; attributes can themselves contain sub-values that can be accessed from the query language. For example, you can create attributes that are arrays of base types.

Arrays

Postgres 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.

CREATE TABLE SAL_EMP (
    name            text,
    pay_by_quarter  int4[],
    schedule        text[][]
);

The above query will create a class named SAL_EMP with a text string (name), a one-dimensional array of int4 (pay_by_quarter), which represents the employee's salary by quarter and a two-dimensional array of text (schedule), which represents the employee's weekly schedule. Now we do some INSERTSs; note that when appending to an array, we enclose the values within braces and separate them by commas. If you know C, this is not unlike the syntax for initializing structures.

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"}}');

By default, Postgres 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:

SELECT name
    FROM SAL_EMP
    WHERE SAL_EMP.pay_by_quarter[1] <>
    SAL_EMP.pay_by_quarter[2];

+------+
|name  |
+------+
|Carol |
+------+

This query retrieves the third quarter pay of all employees:

SELECT SAL_EMP.pay_by_quarter[3] FROM SAL_EMP;


+---------------+
|pay_by_quarter |
+---------------+
|10000          |
+---------------+
|25000          |
+---------------+

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.

SELECT SAL_EMP.schedule[1:2][1:1]
    FROM SAL_EMP
    WHERE SAL_EMP.name = 'Bill';

+-------------------+
|schedule           |
+-------------------+
|{{"meeting"},{""}} |
+-------------------+