SQL Server 2005 : Data Querying and Reporting (part 1)

Listing the Contents of a Table

You will often be retrieving all the data from a particular table. Even if the final query is not intended to get all the data, you can often begin the data analysis by examining all the rows and columns of data in a particular table.

SELECT * FROM YUKONTWO.ONE.dbo.Customers

Note that the * is used to obtain all columns from the Customers table. It is also worth noting the use of the four-part name (Server.Owner.Database.Object). This name includes the server name YUKONTWO, database name ONE, the owner name dbo, and the name of the table itself, Customers.

Four-part names are used to perform queries when the one-part name of a table does not sufficiently qualify the table being queried. If you are executing a query within the scope of the server itself with the ONE database in use, and you are the owner or the owner is dbo, the four-part name is not necessary. There are therefore several valid variations on queries for the Customers table. Each of the following will produce the same results:

SELECT * FROM Customers
SELECT * FROM dbo.Customers
SELECT * FROM ONE.dbo.Customers
SELECT * FROM ONE..Customers

Although queries often go after all the data in a table, there are a considerable number of options available for a query statement. You can choose some of the columns of a table, provide record-level conditions to limit the number of rows returned, put the output into groups, provide group-level conditions to selectively choose the groups, put the output into sorted order, or produce calculated results. You can also get into some complex queries through the use of JOIN, UNION, and subquery operations.

You do not need to formulate complex queries for the 70-431 exam, so this chapter covers only the basic theory of the use of the queries, which is what the exam focuses on.

You can optionally supply column headers to give a user-friendly listing of the data. By default, the column headers that are displayed in the result set are the same as the columns specified in the column select list, such as CUSTNMBR and CNTCPRSN.

Making a Report More Presentable

Why not change a result set’s column header to something more readable? You can change the name of a result set column by specifying the keyword AS. (This is the traditional SQL-92 ANSI standard.) Changing the column name by adding an equals sign (=) or implied assignment is also an alternative syntax choice. (Of course, you would normally use only one of these three techniques, and the industry standard is SQL-92 ANSI.) The following example illustrates the use of column aliasing:

SELECT CUSTNMBR AS 'Customer Number',
      'Customer Name' = CUSTNAME,
      CNTCPRSN 'Contact Person',
      FROM Employees

SELECT CUSTNMBR AS 'Employee ID',
      CUSTNAME AS 'Customer Name',
      CNTCPRSN AS 'Contact Person',
      FROM Employees

Notice that the previous column aliases have been enclosed in single quotation marks. This enclosure is necessary when the column alias includes a space. The alias name must be enclosed within brackets when the alias is a reserved SQL Server keyword.

Sometimes you need to combine two columns together to show the two columns as one. When you do this, you are using a method called string concatenation. You can think of concatenation as joining strings together, just as you can combine words into phrases. The operator used to perform the concatenation is the plus sign (+).

Using TRIM to Remove White Space

To create a single name column, you combine the last name and first name values. If there are leading or training blanks within the data, you might want to polish the output a little bit, by using the functions LTRIM (left trim) and RTRIM (right trim). These functions remove leading spaces (LTRIM) or trailing spaces (RTRIM). The resulting code would then look like this:

SELECT LTRIM(RTRIM(Address1))
 + ' ' + LTRIM(RTRIM(Address2))
 + ' ' + LTRIM(RTRIM(Address3)) 'Full Address'
 FROM Customers

You can create your own function, which you could name TRIM, to eliminate both left and right spaces. This would be a handy feature to have in the product, and here is how it would look:

CREATE FUNCTION dbo.TRIM(@CHARSTRING NVARCHAR(255))
 RETURNS NVARCHAR(255)
 AS
  BEGIN
   RETURN (RTRIM(LTRIM(@CHARSTRING)))
  END

Returning TOP Rows

The TOP clause limits the number of rows returned in a result set to a specified number or percentage at the top of a sorted range. Here are two examples:

• SELECT TOP 50 returns the top 50 rows.

• SELECT TOP 50 PERCENT returns the top 50% of the rows.

As an alternative to TOP, you can also limit the number of rows to return by using SET ROWCOUNT N. The difference between this keyword and TOP is that the TOP keyword applies to the single SELECT statement in which it is specified. For example, SET ROWCOUNT stays in effect until another SET ROWCOUNTSET ROWCOUNT 0 to turn off the option). statement is executed (for example,

You can optionally specify that the TOP keyword is to use the WITH TIES option. In this case, any number of records can possibly be displayed. WITH TIES displays all records that are equivalent to the last matching element. If you are looking for the top 10 employees and two employees tie for 10th, 11 or more records are displayed. If the tie is for 9th or a higher position, only 10 records are listed.

Of course, after you begin placing data in the desired order, you then need to group the output and perform calculations based on the groups. As discussed in the following section, grouping allows the production of subtotals and also provides more usable output in applications that require grouped output.

Displaying Groups in Output

You can use the GROUP BY clause of the SELECT statement to create groups within data. You can then use these groups to display data in a more orderly fashion or produce more meaningful results through the use of aggregate functions.

The GROUP BY clause specifies the groups into which output is to be shown and, if aggregate functions are included, calculations of summary values are performed for each group. When GROUP BY is specified, either each column in any non-aggregate expression in the select list should be included in the GROUP BY list, or the GROUP BY expression must match exactly the select list expression.

It is important to note that if the ORDER BY clause is not specified, groups returned using the GROUP BY clause are not in any particular order. It is recommended that you always use the ORDER BY clause to specify a particular ordering of data. Data will still be collected into groups. See the following example:

SELECT Country, Count(DISTINCT City) AS 'Number of Cities'
 FROM Customers GROUP BY Country

In this example, countries are collected together and are placed in the order chosen by SQL Server (usually ascending). The number of unique cities is counted and displayed beside the related country. By supplying the ORDER BY clause, as in the following example, you sort data into descending sequence, placing the country with the greatest number of unique cities at the top:

SELECT Country, Count(DISTINCT City) AS 'Number of Cities'
 FROM Customers GROUP BY Country
 ORDER BY Count(DISTINCT City) DESC

You might not want all groups to be included in the output. To exclude groups from the recordset, you can utilize the HAVING clause, which operates against the groups of data in the same way that the WHERE clause acts against the individual rows. In the example shown in Figure 1, the listing has been narrowed down through the elimination of countries with fewer than three unique cities.

Figure 1. An example of GROUP, HAVING, and ORDER used together.

The HAVING clause is similar to the WHERE clause. In a SELECT statement, these clauses control the rows from the source tables that are used to build the result set. WHERE and HAVING are filters: They specify a series of search conditions, and only those rows that meet the terms of the search conditions are used to build the result set. To address how these clauses are used, you must understand the conditions that can be applied within these clauses.