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- <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>5.1. Table Basics</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets V1.79.1" /><link rel="prev" href="ddl.html" title="Chapter 5. Data Definition" /><link rel="next" href="ddl-default.html" title="5.2. Default Values" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">5.1. Table Basics</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="ddl.html" title="Chapter 5. Data Definition">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="ddl.html" title="Chapter 5. Data Definition">Up</a></td><th width="60%" align="center">Chapter 5. Data Definition</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="ddl-default.html" title="5.2. Default Values">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="DDL-BASICS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">5.1. Table Basics</h2></div></div></div><a id="id-1.5.4.3.2" class="indexterm"></a><a id="id-1.5.4.3.3" class="indexterm"></a><a id="id-1.5.4.3.4" class="indexterm"></a><p>
- A table in a relational database is much like a table on paper: It
- consists of rows and columns. The number and order of the columns
- is fixed, and each column has a name. The number of rows is
- variable — it reflects how much data is stored at a given moment.
- SQL does not make any guarantees about the order of the rows in a
- table. When a table is read, the rows will appear in an unspecified order,
- unless sorting is explicitly requested. This is covered in <a class="xref" href="queries.html" title="Chapter 7. Queries">Chapter 7</a>. Furthermore, SQL does not assign unique
- identifiers to rows, so it is possible to have several completely
- identical rows in a table. This is a consequence of the
- mathematical model that underlies SQL but is usually not desirable.
- Later in this chapter we will see how to deal with this issue.
- </p><p>
- Each column has a data type. The data type constrains the set of
- possible values that can be assigned to a column and assigns
- semantics to the data stored in the column so that it can be used
- for computations. For instance, a column declared to be of a
- numerical type will not accept arbitrary text strings, and the data
- stored in such a column can be used for mathematical computations.
- By contrast, a column declared to be of a character string type
- will accept almost any kind of data but it does not lend itself to
- mathematical calculations, although other operations such as string
- concatenation are available.
- </p><p>
- <span class="productname">PostgreSQL</span> includes a sizable set of
- built-in data types that fit many applications. Users can also
- define their own data types. Most built-in data types have obvious
- names and semantics, so we defer a detailed explanation to <a class="xref" href="datatype.html" title="Chapter 8. Data Types">Chapter 8</a>. Some of the frequently used data types are
- <code class="type">integer</code> for whole numbers, <code class="type">numeric</code> for
- possibly fractional numbers, <code class="type">text</code> for character
- strings, <code class="type">date</code> for dates, <code class="type">time</code> for
- time-of-day values, and <code class="type">timestamp</code> for values
- containing both date and time.
- </p><a id="id-1.5.4.3.8" class="indexterm"></a><p>
- To create a table, you use the aptly named <a class="xref" href="sql-createtable.html" title="CREATE TABLE"><span class="refentrytitle">CREATE TABLE</span></a> command.
- In this command you specify at least a name for the new table, the
- names of the columns and the data type of each column. For
- example:
- </p><pre class="programlisting">
- CREATE TABLE my_first_table (
- first_column text,
- second_column integer
- );
- </pre><p>
- This creates a table named <code class="literal">my_first_table</code> with
- two columns. The first column is named
- <code class="literal">first_column</code> and has a data type of
- <code class="type">text</code>; the second column has the name
- <code class="literal">second_column</code> and the type <code class="type">integer</code>.
- The table and column names follow the identifier syntax explained
- in <a class="xref" href="sql-syntax-lexical.html#SQL-SYNTAX-IDENTIFIERS" title="4.1.1. Identifiers and Key Words">Section 4.1.1</a>. The type names are
- usually also identifiers, but there are some exceptions. Note that the
- column list is comma-separated and surrounded by parentheses.
- </p><p>
- Of course, the previous example was heavily contrived. Normally,
- you would give names to your tables and columns that convey what
- kind of data they store. So let's look at a more realistic
- example:
- </p><pre class="programlisting">
- CREATE TABLE products (
- product_no integer,
- name text,
- price numeric
- );
- </pre><p>
- (The <code class="type">numeric</code> type can store fractional components, as
- would be typical of monetary amounts.)
- </p><div class="tip"><h3 class="title">Tip</h3><p>
- When you create many interrelated tables it is wise to choose a
- consistent naming pattern for the tables and columns. For
- instance, there is a choice of using singular or plural nouns for
- table names, both of which are favored by some theorist or other.
- </p></div><p>
- There is a limit on how many columns a table can contain.
- Depending on the column types, it is between 250 and 1600.
- However, defining a table with anywhere near this many columns is
- highly unusual and often a questionable design.
- </p><a id="id-1.5.4.3.13" class="indexterm"></a><p>
- If you no longer need a table, you can remove it using the <a class="xref" href="sql-droptable.html" title="DROP TABLE"><span class="refentrytitle">DROP TABLE</span></a> command.
- For example:
- </p><pre class="programlisting">
- DROP TABLE my_first_table;
- DROP TABLE products;
- </pre><p>
- Attempting to drop a table that does not exist is an error.
- Nevertheless, it is common in SQL script files to unconditionally
- try to drop each table before creating it, ignoring any error
- messages, so that the script works whether or not the table exists.
- (If you like, you can use the <code class="literal">DROP TABLE IF EXISTS</code> variant
- to avoid the error messages, but this is not standard SQL.)
- </p><p>
- If you need to modify a table that already exists, see <a class="xref" href="ddl-alter.html" title="5.6. Modifying Tables">Section 5.6</a> later in this chapter.
- </p><p>
- With the tools discussed so far you can create fully functional
- tables. The remainder of this chapter is concerned with adding
- features to the table definition to ensure data integrity,
- security, or convenience. If you are eager to fill your tables with
- data now you can skip ahead to <a class="xref" href="dml.html" title="Chapter 6. Data Manipulation">Chapter 6</a> and read the
- rest of this chapter later.
- </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ddl.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ddl.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ddl-default.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 5. Data Definition </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 5.2. Default Values</td></tr></table></div></body></html>
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