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4. THE QUERY LANGUAGE
The POSTGRES query language is a variant of SQL-3. 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 [MELT93] or
[DATE93]. You should also
be aware that some features are not part of the ANSI
standard.
In the examples that follow, we assume that you have
created the mydb database as described in the previous
subsection and have started psql.
Examples in this manual can also be found in
/usr/local/postgres95/src/tutorial. Refer to the
README file in that directory for how to use them. To
start the tutorial, do the following:
% cd /usr/local/postgres95/src/tutorial
% psql -s mydb
Welcome to the POSTGRES95 interactive sql monitor:
type \? for help on slash commands
type \q to quit
type \g or terminate with semicolon to execute query
You are currently connected to the database: jolly
mydb=> \i basics.sql
The \i command read in queries from the specified
files. The -s option puts you in single step mode which
pauses before sending a query to the backend. Queries
in this section are in the file basics.sql.
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, a row is an
instance and 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.
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 but identifiers
are case-sensitive. POSTGRES SQL supports the usual
SQL types int, float, real, smallint, char(N),
varchar(N), date, and time. As we will
see later, POSTGRES can be customized with an
arbitrary number of
user-defined data types. Consequently, type names are
not keywords.
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.
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.
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 aribitrary 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;
Any select query can be redirected to a new class
SELECT * INTO temp from weather;
This creates 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.
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.2 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 |
+--------------+---------+---------+---------------+---------+---------+
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.3
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 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, the delete command will simply
delete all instances of the given class, leaving it
empty. The system will not request confirmation before
doing this.
Like most other query languages, POSTGRES supports
aggregate functions. However, the current
implementation of POSTGRES aggregate functions is very limited.
Specifically, while there are aggregates to compute
such functions as the count, sum, average, maximum and
minimum over a set of instances, aggregates can only
appear in the target list of a query and not in the
qualification ( where clause) As an example,
SELECT max(temp_lo)
FROM weather;
Aggregates may also have GROUP BY clauses:
SELECT city, max(temp_lo)
FROM weather
GROUP BY city;
2. This is only a conceptual model. The actual join may
be performed in a more efficient manner, but this is invisible to the user.
3. 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.
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