<!-- $Header: /cvsroot/pgsql/doc/src/sgml/query.sgml,v 1.34 2003/11/01 01:56:29 petere Exp $ --> <chapter id="tutorial-sql"> <title>The <acronym>SQL</acronym> Language</title> <sect1 id="tutorial-sql-intro"> <title>Introduction</title> <para> This chapter provides an overview of how to use <acronym>SQL</acronym> to perform simple operations. This tutorial is only intended to give you an introduction and is in no way a complete tutorial on <acronym>SQL</acronym>. Numerous books have been written on <acronym>SQL</acronym>, including <xref linkend="MELT93"> and <xref linkend="DATE97">. You should be aware that some <productname>PostgreSQL</productname> language features are extensions to the standard. </para> <para> In the examples that follow, we assume that you have created a database named <literal>mydb</literal>, as described in the previous chapter, and have started <application>psql</application>. </para> <para> Examples in this manual can also be found in the <productname>PostgreSQL</productname> source distribution in the directory <filename>src/tutorial/</filename>. Refer to the <filename>README</filename> file in that directory for how to use them. To start the tutorial, do the following: <screen> <prompt>$</prompt> <userinput>cd <replaceable>....</replaceable>/src/tutorial</userinput> <prompt>$</prompt> <userinput>psql -s mydb</userinput> <computeroutput> ... </computeroutput> <prompt>mydb=></prompt> <userinput>\i basics.sql</userinput> </screen> The <literal>\i</literal> command reads in commands from the specified file. The <literal>-s</literal> option puts you in single step mode which pauses before sending each statement to the server. The commands used in this section are in the file <filename>basics.sql</filename>. </para> </sect1> <sect1 id="tutorial-concepts"> <title>Concepts</title> <para> <indexterm><primary>relational database</primary></indexterm> <indexterm><primary>hierarchical database</primary></indexterm> <indexterm><primary>object-oriented database</primary></indexterm> <indexterm><primary>relation</primary></indexterm> <indexterm><primary>table</primary></indexterm> <productname>PostgreSQL</productname> is a <firstterm>relational database management system</firstterm> (<acronym>RDBMS</acronym>). That means it is a system for managing data stored in <firstterm>relations</firstterm>. Relation is essentially a mathematical term for <firstterm>table</firstterm>. The notion of storing data in tables is so commonplace today that it might seem inherently obvious, but there are a number of other ways of organizing databases. Files and directories on Unix-like operating systems form an example of a hierarchical database. A more modern development is the object-oriented database. </para> <para> <indexterm><primary>row</primary></indexterm> <indexterm><primary>column</primary></indexterm> Each table is a named collection of <firstterm>rows</firstterm>. Each row of a given table has the same set of named <firstterm>columns</firstterm>, and each column is of a specific data type. Whereas columns have a fixed order in each row, it is important to remember that SQL does not guarantee the order of the rows within the table in any way (although they can be explicitly sorted for display). </para> <para> <indexterm><primary>database cluster</primary></indexterm> <indexterm><primary>cluster</primary><secondary>of databases</secondary><see>database cluster</see></indexterm> Tables are grouped into databases, and a collection of databases managed by a single <productname>PostgreSQL</productname> server instance constitutes a database <firstterm>cluster</firstterm>. </para> </sect1> <sect1 id="tutorial-table"> <title>Creating a New Table</title> <indexterm zone="tutorial-table"> <primary>CREATE TABLE</primary> </indexterm> <para> You can create a new table by specifying the table name, along with all column names and their types: <programlisting> CREATE TABLE weather ( city varchar(80), temp_lo int, -- low temperature temp_hi int, -- high temperature prcp real, -- precipitation date date ); </programlisting> You can enter this into <command>psql</command> with the line breaks. <command>psql</command> will recognize that the command is not terminated until the semicolon. </para> <para> White space (i.e., spaces, tabs, and newlines) may be used freely in SQL commands. That means you can type the command aligned differently than above, or even all on one line. Two dashes (<quote><literal>--</literal></quote>) introduce comments. Whatever follows them is ignored up to the end of the line. SQL is case insensitive about key words and identifiers, except when identifiers are double-quoted to preserve the case (not done above). </para> <para> <type>varchar(80)</type> specifies a data type that can store arbitrary character strings up to 80 characters in length. <type>int</type> is the normal integer type. <type>real</type> is a type for storing single precision floating-point numbers. <type>date</type> should be self-explanatory. (Yes, the column of type <type>date</type> is also named <literal>date</literal>. This may be convenient or confusing -- you choose.) </para> <para> <productname>PostgreSQL</productname> supports the usual <acronym>SQL</acronym> types <type>int</type>, <type>smallint</type>, <type>real</type>, <type>double precision</type>, <type>char(<replaceable>N</>)</type>, <type>varchar(<replaceable>N</>)</type>, <type>date</type>, <type>time</type>, <type>timestamp</type>, and <type>interval</type>, as well as other types of general utility and a rich set of geometric types. <productname>PostgreSQL</productname> can be customized with an arbitrary number of user-defined data types. Consequently, type names are not syntactical key words, except where required to support special cases in the <acronym>SQL</acronym> standard. </para> <para> The second example will store cities and their associated geographical location: <programlisting> CREATE TABLE cities ( name varchar(80), location point ); </programlisting> The <type>point</type> type is an example of a <productname>PostgreSQL</productname>-specific data type. </para> <para> <indexterm> <primary>DROP TABLE</primary> </indexterm> Finally, it should be mentioned that if you don't need a table any longer or want to recreate it differently you can remove it using the following command: <synopsis> DROP TABLE <replaceable>tablename</replaceable>; </synopsis> </para> </sect1> <sect1 id="tutorial-populate"> <title>Populating a Table With Rows</title> <indexterm zone="tutorial-populate"> <primary>INSERT</primary> </indexterm> <para> The <command>INSERT</command> statement is used to populate a table with rows: <programlisting> INSERT INTO weather VALUES ('San Francisco', 46, 50, 0.25, '1994-11-27'); </programlisting> Note that all data types use rather obvious input formats. Constants that are not simple numeric values usually must be surrounded by single quotes (<literal>'</>), as in the example. The <type>date</type> type is actually quite flexible in what it accepts, but for this tutorial we will stick to the unambiguous format shown here. </para> <para> The <type>point</type> type requires a coordinate pair as input, as shown here: <programlisting> INSERT INTO cities VALUES ('San Francisco', '(-194.0, 53.0)'); </programlisting> </para> <para> The syntax used so far requires you to remember the order of the columns. An alternative syntax allows you to list the columns explicitly: <programlisting> INSERT INTO weather (city, temp_lo, temp_hi, prcp, date) VALUES ('San Francisco', 43, 57, 0.0, '1994-11-29'); </programlisting> You can list the columns in a different order if you wish or even omit some columns, e.g., if the precipitation is unknown: <programlisting> INSERT INTO weather (date, city, temp_hi, temp_lo) VALUES ('1994-11-29', 'Hayward', 54, 37); </programlisting> Many developers consider explicitly listing the columns better style than relying on the order implicitly. </para> <para> Please enter all the commands shown above so you have some data to work with in the following sections. </para> <para> <indexterm> <primary>COPY</primary> </indexterm> You could also have used <command>COPY</command> to load large amounts of data from flat-text files. This is usually faster because the <command>COPY</command> command is optimized for this application while allowing less flexibility than <command>INSERT</command>. An example would be: <programlisting> COPY weather FROM '/home/user/weather.txt'; </programlisting> where the file name for the source file must be available to the backend server machine, not the client, since the backend server reads the file directly. You can read more about the <command>COPY</command> command in <xref linkend="sql-copy">. </para> </sect1> <sect1 id="tutorial-select"> <title>Querying a Table</title> <para> <indexterm><primary>query</primary></indexterm> <indexterm><primary>SELECT</primary></indexterm> To retrieve data from a table, the table is <firstterm>queried</firstterm>. An <acronym>SQL</acronym> <command>SELECT</command> statement is used to do this. The statement is divided into a select list (the part that lists the columns to be returned), a table list (the part that lists the tables from which to retrieve the data), and an optional qualification (the part that specifies any restrictions). For example, to retrieve all the rows of table <classname>weather</classname>, type: <programlisting> SELECT * FROM weather; </programlisting> (here <literal>*</literal> means <quote>all columns</quote>) and the output should be: <screen> city | temp_lo | temp_hi | prcp | date ---------------+---------+---------+------+------------ San Francisco | 46 | 50 | 0.25 | 1994-11-27 San Francisco | 43 | 57 | 0 | 1994-11-29 Hayward | 37 | 54 | | 1994-11-29 (3 rows) </screen> </para> <para> You may specify any arbitrary expressions in the select list. For example, you can do: <programlisting> SELECT city, (temp_hi+temp_lo)/2 AS temp_avg, date FROM weather; </programlisting> This should give: <screen> city | temp_avg | date ---------------+----------+------------ San Francisco | 48 | 1994-11-27 San Francisco | 50 | 1994-11-29 Hayward | 45 | 1994-11-29 (3 rows) </screen> Notice how the <literal>AS</literal> clause is used to relabel the output column. (It is optional.) </para> <para> Arbitrary Boolean operators (<literal>AND</literal>, <literal>OR</literal>, and <literal>NOT</literal>) are allowed in the qualification of a query. For example, the following retrieves the weather of San Francisco on rainy days: <programlisting> SELECT * FROM weather WHERE city = 'San Francisco' AND prcp > 0.0; </programlisting> Result: <screen> city | temp_lo | temp_hi | prcp | date ---------------+---------+---------+------+------------ San Francisco | 46 | 50 | 0.25 | 1994-11-27 (1 row) </screen> </para> <para> <indexterm><primary>ORDER BY</primary></indexterm> <indexterm><primary>DISTINCT</primary></indexterm> <indexterm><primary>duplicate</primary></indexterm> As a final note, you can request that the results of a query can be returned in sorted order or with duplicate rows removed: <programlisting> SELECT DISTINCT city FROM weather ORDER BY city; </programlisting> <screen> city --------------- Hayward San Francisco (2 rows) </screen> <literal>DISTINCT</literal> and <literal>ORDER BY</literal> can be used separately, of course. </para> </sect1> <sect1 id="tutorial-join"> <title>Joins Between Tables</title> <indexterm zone="tutorial-join"> <primary>join</primary> </indexterm> <para> Thus far, our queries have only accessed one table at a time. Queries can access multiple tables at once, or access the same table in such a way that multiple rows of the table are being processed at the same time. A query that accesses multiple rows of the same or different tables at one time is called a <firstterm>join</firstterm> query. As an example, say you wish to list all the weather records together with the location of the associated city. To do that, we need to compare the city column of each row of the weather table with the name column of all rows in the cities table, and select the pairs of rows where these values match. <note> <para> This is only a conceptual model. The actual join may be performed in a more efficient manner, but this is invisible to the user. </para> </note> This would be accomplished by the following query: <programlisting> SELECT * FROM weather, cities WHERE city = name; </programlisting> <screen> city | temp_lo | temp_hi | prcp | date | name | location ---------------+---------+---------+------+------------+---------------+----------- San Francisco | 46 | 50 | 0.25 | 1994-11-27 | San Francisco | (-194,53) San Francisco | 43 | 57 | 0 | 1994-11-29 | San Francisco | (-194,53) (2 rows) </screen> </para> <para> Observe two things about the result set: <itemizedlist> <listitem> <para> There is no result row for the city of Hayward. This is because there is no matching entry in the <classname>cities</classname> table for Hayward, so the join ignores the unmatched rows in the weather table. We will see shortly how this can be fixed. </para> </listitem> <listitem> <para> There are two columns containing the city name. This is correct because the lists of columns of the <classname>weather</classname> and the <classname>cities</classname> table are concatenated. In practice this is undesirable, though, so you will probably want to list the output columns explicitly rather than using <literal>*</literal>: <programlisting> SELECT city, temp_lo, temp_hi, prcp, date, location FROM weather, cities WHERE city = name; </programlisting> </para> </listitem> </itemizedlist> </para> <formalpara> <title>Exercise:</title> <para> Attempt to find out the semantics of this query when the <literal>WHERE</literal> clause is omitted. </para> </formalpara> <para> Since the columns all had different names, the parser automatically found out which table they belong to, but it is good style to fully qualify column names in join queries: <programlisting> SELECT weather.city, weather.temp_lo, weather.temp_hi, weather.prcp, weather.date, cities.location FROM weather, cities WHERE cities.name = weather.city; </programlisting> </para> <para> Join queries of the kind seen thus far can also be written in this alternative form: <programlisting> SELECT * FROM weather INNER JOIN cities ON (weather.city = cities.name); </programlisting> This syntax is not as commonly used as the one above, but we show it here to help you understand the following topics. </para> <para> <indexterm><primary>join</primary><secondary>outer</secondary></indexterm> Now we will figure out how we can get the Hayward records back in. What we want the query to do is to scan the <classname>weather</classname> table and for each row to find the matching <classname>cities</classname> row. If no matching row is found we want some <quote>empty values</quote> to be substituted for the <classname>cities</classname> table's columns. This kind of query is called an <firstterm>outer join</firstterm>. (The joins we have seen so far are inner joins.) The command looks like this: <programlisting> SELECT * FROM weather LEFT OUTER JOIN cities ON (weather.city = cities.name); city | temp_lo | temp_hi | prcp | date | name | location ---------------+---------+---------+------+------------+---------------+----------- Hayward | 37 | 54 | | 1994-11-29 | | San Francisco | 46 | 50 | 0.25 | 1994-11-27 | San Francisco | (-194,53) San Francisco | 43 | 57 | 0 | 1994-11-29 | San Francisco | (-194,53) (3 rows) </programlisting> This query is called a <firstterm>left outer join</firstterm> because the table mentioned on the left of the join operator will have each of its rows in the output at least once, whereas the table on the right will only have those rows output that match some row of the left table. When outputting a left-table row for which there is no right-table match, empty (null) values are substituted for the right-table columns. </para> <formalpara> <title>Exercise:</title> <para> There are also right outer joins and full outer joins. Try to find out what those do. </para> </formalpara> <para> <indexterm><primary>join</primary><secondary>self</secondary></indexterm> <indexterm><primary>alias</primary><secondary>for table name in query</secondary></indexterm> We can also join a table against itself. This is called a <firstterm>self join</firstterm>. As an example, suppose we wish to find all the weather records that are in the temperature range of other weather records. So we need to compare the <structfield>temp_lo</> and <structfield>temp_hi</> columns of each <classname>weather</classname> row to the <structfield>temp_lo</structfield> and <structfield>temp_hi</structfield> columns of all other <classname>weather</classname> rows. We can do this with the following query: <programlisting> SELECT W1.city, W1.temp_lo AS low, W1.temp_hi AS high, W2.city, W2.temp_lo AS low, W2.temp_hi AS high FROM weather W1, weather W2 WHERE W1.temp_lo < W2.temp_lo AND W1.temp_hi > W2.temp_hi; city | low | high | city | low | high ---------------+-----+------+---------------+-----+------ San Francisco | 43 | 57 | San Francisco | 46 | 50 Hayward | 37 | 54 | San Francisco | 46 | 50 (2 rows) </programlisting> Here we have relabeled the weather table as <literal>W1</> and <literal>W2</> to be able to distinguish the left and right side of the join. You can also use these kinds of aliases in other queries to save some typing, e.g.: <programlisting> SELECT * FROM weather w, cities c WHERE w.city = c.name; </programlisting> You will encounter this style of abbreviating quite frequently. </para> </sect1> <sect1 id="tutorial-agg"> <title>Aggregate Functions</title> <indexterm zone="tutorial-agg"> <primary>aggregate function</primary> </indexterm> <para> <indexterm><primary>average</primary></indexterm> <indexterm><primary>count</primary></indexterm> <indexterm><primary>max</primary></indexterm> <indexterm><primary>min</primary></indexterm> <indexterm><primary>sum</primary></indexterm> Like most other relational database products, <productname>PostgreSQL</productname> supports aggregate functions. An aggregate function computes a single result from multiple input rows. For example, there are aggregates to compute the <function>count</function>, <function>sum</function>, <function>avg</function> (average), <function>max</function> (maximum) and <function>min</function> (minimum) over a set of rows. </para> <para> As an example, we can find the highest low-temperature reading anywhere with <programlisting> SELECT max(temp_lo) FROM weather; </programlisting> <screen> max ----- 46 (1 row) </screen> </para> <para> <indexterm><primary>subquery</primary></indexterm> If we wanted to know what city (or cities) that reading occurred in, we might try <programlisting> SELECT city FROM weather WHERE temp_lo = max(temp_lo); <lineannotation>WRONG</lineannotation> </programlisting> but this will not work since the aggregate <function>max</function> cannot be used in the <literal>WHERE</literal> clause. (This restriction exists because the <literal>WHERE</literal> clause determines the rows that will go into the aggregation stage; so it has to be evaluated before aggregate functions are computed.) However, as is often the case the query can be restated to accomplish the intended result, here by using a <firstterm>subquery</firstterm>: <programlisting> SELECT city FROM weather WHERE temp_lo = (SELECT max(temp_lo) FROM weather); </programlisting> <screen> city --------------- San Francisco (1 row) </screen> This is OK because the subquery is an independent computation that computes its own aggregate separately from what is happening in the outer query. </para> <para> <indexterm><primary>GROUP BY</primary></indexterm> <indexterm><primary>HAVING</primary></indexterm> Aggregates are also very useful in combination with <literal>GROUP BY</literal> clauses. For example, we can get the maximum low temperature observed in each city with <programlisting> SELECT city, max(temp_lo) FROM weather GROUP BY city; </programlisting> <screen> city | max ---------------+----- Hayward | 37 San Francisco | 46 (2 rows) </screen> which gives us one output row per city. Each aggregate result is computed over the table rows matching that city. We can filter these grouped rows using <literal>HAVING</literal>: <programlisting> SELECT city, max(temp_lo) FROM weather GROUP BY city HAVING max(temp_lo) < 40; </programlisting> <screen> city | max ---------+----- Hayward | 37 (1 row) </screen> which gives us the same results for only the cities that have all <literal>temp_lo</> values below 40. Finally, if we only care about cities whose names begin with <quote><literal>S</literal></quote>, we might do <programlisting> SELECT city, max(temp_lo) FROM weather WHERE city LIKE 'S%'<co id="co.tutorial-agg-like"> GROUP BY city HAVING max(temp_lo) < 40; </programlisting> <calloutlist> <callout arearefs="co.tutorial-agg-like"> <para> The <literal>LIKE</literal> operator does pattern matching and is explained in <xref linkend="functions-matching">. </para> </callout> </calloutlist> </para> <para> It is important to understand the interaction between aggregates and <acronym>SQL</acronym>'s <literal>WHERE</literal> and <literal>HAVING</literal> clauses. The fundamental difference between <literal>WHERE</literal> and <literal>HAVING</literal> is this: <literal>WHERE</literal> selects input rows before groups and aggregates are computed (thus, it controls which rows go into the aggregate computation), whereas <literal>HAVING</literal> selects group rows after groups and aggregates are computed. Thus, the <literal>WHERE</literal> clause must not contain aggregate functions; it makes no sense to try to use an aggregate to determine which rows will be inputs to the aggregates. On the other hand, <literal>HAVING</literal> clause always contains aggregate functions. (Strictly speaking, you are allowed to write a <literal>HAVING</literal> clause that doesn't use aggregates, but it's wasteful: The same condition could be used more efficiently at the <literal>WHERE</literal> stage.) </para> <para> Observe that we can apply the city name restriction in <literal>WHERE</literal>, since it needs no aggregate. This is more efficient than adding the restriction to <literal>HAVING</literal>, because we avoid doing the grouping and aggregate calculations for all rows that fail the <literal>WHERE</literal> check. </para> </sect1> <sect1 id="tutorial-update"> <title>Updates</title> <indexterm zone="tutorial-update"> <primary>UPDATE</primary> </indexterm> <para> You can update existing rows using the <command>UPDATE</command> command. Suppose you discover the temperature readings are all off by 2 degrees as of November 28. You may update the data as follows: <programlisting> UPDATE weather SET temp_hi = temp_hi - 2, temp_lo = temp_lo - 2 WHERE date > '1994-11-28'; </programlisting> </para> <para> Look at the new state of the data: <programlisting> SELECT * FROM weather; city | temp_lo | temp_hi | prcp | date ---------------+---------+---------+------+------------ San Francisco | 46 | 50 | 0.25 | 1994-11-27 San Francisco | 41 | 55 | 0 | 1994-11-29 Hayward | 35 | 52 | | 1994-11-29 (3 rows) </programlisting> </para> </sect1> <sect1 id="tutorial-delete"> <title>Deletions</title> <indexterm zone="tutorial-delete"> <primary>DELETE</primary> </indexterm> <para> Suppose you are no longer interested in the weather of Hayward. Then you can do the following to delete those rows from the table. Deletions are performed using the <command>DELETE</command> command: <programlisting> DELETE FROM weather WHERE city = 'Hayward'; </programlisting> All weather records belonging to Hayward are removed. <programlisting> SELECT * FROM weather; </programlisting> <screen> city | temp_lo | temp_hi | prcp | date ---------------+---------+---------+------+------------ San Francisco | 46 | 50 | 0.25 | 1994-11-27 San Francisco | 41 | 55 | 0 | 1994-11-29 (2 rows) </screen> </para> <para> One should be wary of statements of the form <synopsis> DELETE FROM <replaceable>tablename</replaceable>; </synopsis> Without a qualification, <command>DELETE</command> will remove <emphasis>all</> rows from the given table, leaving it empty. The system will not request confirmation before doing this! </para> </sect1> </chapter> <!-- Keep this comment at the end of the file Local variables: mode:sgml sgml-omittag:nil sgml-shorttag:t sgml-minimize-attributes:nil sgml-always-quote-attributes:t sgml-indent-step:1 sgml-indent-data:t sgml-parent-document:nil sgml-default-dtd-file:"./reference.ced" sgml-exposed-tags:nil sgml-local-catalogs:("/usr/lib/sgml/catalog") sgml-local-ecat-files:nil End: -->