Tables, rows, columns, and data values

A table has a defined set of columns. Each column has a name and SQL Server data type. Every row represents one record, and every column represents one attribute of that record. A cell may contain a value or NULL. This lesson uses DDI_SQL_Training dataset v1.0.0 on SQL Server 2019 or later.

Read a real table

The Customers table has CustomerID, FirstName, LastName, City, Region, and Email. CustomerID is an integer. Names and locations are Unicode text. Email permits NULL. This short result intentionally selects only four of the six columns. Read it now; you can run it after setting up the practice database in lesson 5.

SELECT TOP (3) CustomerID, FirstName, LastName, City
FROM dbo.Customers
ORDER BY CustomerID;

TOP (3) limits this result to three rows. ORDER BY CustomerID gives the returned rows a defined order. We’ll study SELECT and sorting in detail in Chapters 2 and 3; here, focus on the shape of the result.

What NULL means

Customer 5 has a NULL Email. NULL represents a missing or unknown value. It is not the text "NULL", an empty string, or zero. The column definition decides whether a row may contain NULL.

Check your understanding