Sql Excel : What Is a Table?

A table is a set of rows and columns that describe multiple instances of something.
Each row represents one instance—such as a single purchase made by a customer,
or a single visit to a web page, or a single zip code with its demographic details.
Each column contains one attribute for one instance. SQL tables represent
unordered sets, so the table does not have a first row or a last row—unless a
specific column such as an id or creation date provides that information.
Any given column contains the same genre of information for all rows. So a zip
code column should not be the “sent-to” zip code in one row and the “billed-to” zip
code in another. Although these are both zip codes, they represent two different
uses, so they belong in two different columns.
Columns, unless declared otherwise, are permitted to take on the value NULL,
meaning that the value is not available or is unknown. For instance, a row
describing customers might contain a column for birthdate. This column would
take on the value of NULL for all rows where the birthdate is not known.
A table can have as many columns as needed to describe an instance, although for
practical purposes tables with more than a few hundred columns are rare (and
most relational databases do have an upper limit on the number of columns in a
single table, typically in the low thousands). A table can have as many rows as
needed; here the numbers easily rise to the millions and even billions.
As an example, Table 1.1 shows a few rows and columns from ZipCensus (which is
available on the companion website). This table shows that each zip code is
assigned to a particular state, which is the abbreviation in the stab column (“STate
ABbreviation”). The pctstate column is an indicator that zip codes sometimes span
state boundaries. For instance, 10004 is a zip code in New York City that covers
Ellis Island. In 1998, the Supreme Court split jurisdiction of the island between
New York and New Jersey, but the Post Office did not change the zip code. So,
10004 has a portion in New York and a smaller, unpopulated portion in New
Jersey.