The Query Language
This chapter must go into depth on each area of the query language.
Currently a copy of the tutorial.
- thomas 1998-01-12
The Postgres query language is a variant of
SQL3. It
has many extensions such as an extensible type system,
inheritance, functions and production rules. Those are
features carried over from the original Postgres
query
language, PostQuel.
This section provides an overview
of how to use Postgres SQL
to perform simple operations.
This manual is only intended to give you an idea of our
flavor of SQL and is in no way a complete tutorial on
SQL. Numerous books have been written on SQL. For
instance, consult or
. You should also
be aware that some features of Postgres
are not part of the ANSI standard.
Concepts
The fundamental notion in Postgres
is that of a class,
which is a named collection of object instances. Each
instance has the same collection of named attributes,
and each attribute is of a specific type. Furthermore,
each instance has a permanent object
identifier (OID)
that is unique throughout the installation. Because
SQL syntax refers to tables, we will use the terms
table and class interchangeably.
Likewise, an SQL row is an
instance and SQL
columns
are attributes.
As previously discussed, classes are grouped into
databases, and a collection of databases managed by a
single postmaster process constitutes an installation
or site.
Creating a New Class
You can create a new class by specifying the class
name, along with all attribute names and their types:
CREATE TABLE weather (
city varchar(80),
temp_lo int, -- low temperature
temp_hi int, -- high temperature
prcp real, -- precipitation
date date
);
Note that keywords are case-insensitive and identifiers
are usually case-insensitive.
Postgres allows SQL92 delimited identifiers
(identifiers surrounded by double-quotes) to include mixed-case and spaces, tabs, etc.
Postgres SQL supports the usual
SQL types int,
float, real, smallint, char(N),
varchar(N), date, time,
and timestamp, as well as other types of general utility and
a rich set of geometric types. As we will
see later, Postgres can be customized with an
arbitrary number of
user-defined data types. Consequently, type names are
not syntactical keywords, except where required to support special cases in the SQL92 standard.
So far, the Postgres create command looks exactly like
the command used to create a table in a traditional
relational system. However, we will presently see that
classes have properties that are extensions of the
relational model.
Populating a Class with Instances
The insert statement is used to populate a class with
instances:
INSERT INTO weather
VALUES ('San Francisco', 46, 50, 0.25, '11/27/1994')
You can also use the copy command to perform load large
amounts of data from flat (ASCII) files.
This is usually faster because the data is read (or written) as a single atomic
transaction directly to or from the target table. An example would be:
COPY INTO weather FROM '/home/user/weather.txt'
USING DELIMITERS '|';
where the path name for the source file must be available to the backend server
machine, not just the client.
Querying a Class
The weather class can be queried with normal relational
selection and projection queries. A SQL select
statement is used to do this. The statement is divided into
a target list (the part that lists the attributes to be
returned) and a qualification (the part that specifies
any restrictions). For example, to retrieve all the
rows of weather, type:
SELECT * FROM WEATHER;
and the output should be:
+--------------+---------+---------+------+------------+
|city | temp_lo | temp_hi | prcp | date |
+--------------+---------+---------+------+------------+
|San Francisco | 46 | 50 | 0.25 | 11-27-1994 |
+--------------+---------+---------+------+------------+
|San Francisco | 43 | 57 | 0 | 11-29-1994 |
+--------------+---------+---------+------+------------+
|Hayward | 37 | 54 | | 11-29-1994 |
+--------------+---------+---------+------+------------+
You may specify any arbitrary expressions in the target list. For example, you can do:
SELECT city, (temp_hi+temp_lo)/2 AS temp_avg, date FROM weather;
Arbitrary Boolean operators
(and, or and not) are
allowed in the qualification of any query. For example,
SELECT * FROM weather
WHERE city = 'San Francisco'
AND prcp > 0.0;
+--------------+---------+---------+------+------------+
|city | temp_lo | temp_hi | prcp | date |
+--------------+---------+---------+------+------------+
|San Francisco | 46 | 50 | 0.25 | 11-27-1994 |
+--------------+---------+---------+------+------------+
As a final note, you can specify that the results of a
select can be returned in a sorted order
or with duplicate instances removed.
SELECT DISTINCT city
FROM weather
ORDER BY city;
Redirecting SELECT Queries
Any select query can be redirected to a new class
SELECT * INTO TABLE temp FROM weather;
This forms an implicit create command, creating a new
class temp with the attribute names and types specified
in the target list of the select into command. We can
then, of course, perform any operations on the resulting
class that we can perform on other classes.
Joins Between Classes
Thus far, our queries have only accessed one class at a
time. Queries can access multiple classes at once, or
access the same class in such a way that multiple
instances of the class are being processed at the same
time. A query that accesses multiple instances of the
same or different classes at one time is called a join
query.
As an example, say we wish to find all the records that
are in the temperature range of other records. In
effect, we need to compare the temp_lo and temp_hi
attributes of each EMP instance to the temp_lo and
temp_hi attributes of all other EMP instances.
This is only a conceptual model. The actual join may
be performed in a more efficient manner, but this is invisible to the user.
We can do this with the following query:
SELECT W1.city, W1.temp_lo, W1.temp_hi,
W2.city, W2.temp_lo, W2.temp_hi
FROM weather W1, weather W2
WHERE W1.temp_lo < W2.temp_lo
AND W1.temp_hi > W2.temp_hi;
+--------------+---------+---------+---------------+---------+---------+
|city | temp_lo | temp_hi | city | temp_lo | temp_hi |
+--------------+---------+---------+---------------+---------+---------+
|San Francisco | 43 | 57 | San Francisco | 46 | 50 |
+--------------+---------+---------+---------------+---------+---------+
|San Francisco | 37 | 54 | San Francisco | 46 | 50 |
+--------------+---------+---------+---------------+---------+---------+
The semantics of such a join are
that the qualification
is a truth expression defined for the Cartesian product of
the classes indicated in the query. For those instances in
the Cartesian product for which the qualification is true,
Postgres computes and returns the values specified in the
target list. Postgres SQL does not assign any meaning to
duplicate values in such expressions. This means that Postgres
sometimes recomputes the same target list several times;
this frequently happens when Boolean expressions are connected
with an "or". To remove such duplicates, you must use
the select distinct statement.
In this case, both W1 and W2 are surrogates for an
instance of the class weather, and both range over all
instances of the class. (In the terminology of most
database systems, W1 and W2 are known as range variables.)
A query can contain an arbitrary number of
class names and surrogates.
Updates
You can update existing instances using the update command.
Suppose you discover the temperature readings are
all off by 2 degrees as of Nov 28, you may update the
data as follow:
UPDATE weather
SET temp_hi = temp_hi - 2, temp_lo = temp_lo - 2
WHERE date > '11/28/1994';
Deletions
Deletions are performed using the delete command:
DELETE FROM weather WHERE city = 'Hayward';
All weather recording belongs to Hayward is removed.
One should be wary of queries of the form
DELETE FROM classname;
Without a qualification, delete will simply
remove all instances of the given class, leaving it
empty. The system will not request confirmation before
doing this.
Using Aggregate Functions
Like most other query languages, PostgreSQL supports
aggregate functions.
The current implementation of Postgres aggregate functions have some limitations.
Specifically, while there are aggregates to compute
such functions as the count, sum,
avg (average), max (maximum) and
min (minimum) over a set of instances, aggregates can only
appear in the target list of a query and not directly in the
qualification (the where clause). As an example,
SELECT max(temp_lo) FROM weather;
is allowed, while
SELECT city FROM weather WHERE temp_lo = max(temp_lo);
is not. However, as is often the case the query can be restated to accomplish
the intended result; here by using a subselect:
SELECT city FROM weather WHERE temp_lo = (SELECT max(temp_lo) FROM weather);
Aggregates may also have group by clauses:
SELECT city, max(temp_lo)
FROM weather
GROUP BY city;