When You Can't Change a SQL Database Design

Jan 25, 2017

📄 Contents

␡

Item 18: Use Views to Simplify What Cannot Be Changed
Item 19: Use ETL to Turn Nonrelational Data into Information
Item 20: Create Summary Tables and Maintain Them
Item 21: Use UNION Statements to "Unpivot" Non-normalized Data

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Item 21: Use `UNION` Statements to “Unpivot” Non-normalized Data

You saw in Item 3, “Get rid of repeating groups,” how UNION queries can be used to deal with repeating groups. We explore UNION queries a little bit more in this item. As you will learn in Item 22, “Understand relational algebra and how it is implemented in SQL,” the Union operation is one of the eight relational algebra operations that can be performed within the relational model defined by Dr. Edgar F. Codd. It is used to merge data sets created by two (or more) SELECT statements.

Assume that the only way you are able to get some data for analysis is in the form of the Excel spreadsheet pictured in Figure 3.5, which is obviously not normalized.

Figure 3.5 Non-normalized data from Excel

Assuming you can import that data into your DBMS, at best you will end up with a table (SalesSummary) that has five pairs of repeating groups, which we will call OctQuantity, OctSales, NovQuantity, NovSales, and so on to FebQuantity and FebSales.

Listing 3.8 shows a query that would let you look at the October data.

Listing 3.8 SQL to extract October data

SELECT Category, OctQuantity, OctSales
FROM SalesSummary;

Of course, to look at the data for a different month, you need a different query. And let’s not forget that data that is not normalized can be more difficult to use for analysis purposes. This is where a UNION query can help.

There are three basic rules that apply when using UNION queries:

There must be the same number of columns in each of the queries making up the UNION query.
The order of the columns in each of the queries making up the UNION query must be the same.
The data types of the columns in each of the queries must be compatible.

Note that there is nothing in those rules about the names of the columns in the queries that make up the UNION query.

Listing 3.9 shows how to combine all of the data into a normalized view.

Listing 3.9 Using `UNION` to normalize the data

SELECT Category, OctQuantity, OctSales
FROM SalesSummary
UNION
SELECT Category, NovQuantity, NovSales
FROM SalesSummary
UNION
SELECT Category, DecQuantity, DecSales
FROM SalesSummary
UNION
SELECT Category, JanQuantity, JanSales
FROM SalesSummary
UNION
SELECT Category, FebQuantity, FebSales
FROM SalesSummary;

Table 3.2 shows a partial extract of the data returned.

Table 3.2 Partial extract of data returned by the `UNION` query in Listing 3.9

Category	OctQuantity	OctSales
Accessories	923	60883.03
Accessories	930	61165.40
. . .	. . .	. . .
Bikes	450	585130.50
Bikes	542	705733.50
Car racks	96	16772.05
Car racks	115	20137.05
Car racks	124	21763.30
. . .	. . .	. . .
Skateboards	203	89040.58
Skateboards	204	79461.30
Tires	110	3081.24
Tires	137	3937.70
Tires	150	4388.55
Tires	151	4356.91
Tires	186	5377.60

Two things should stand out. First, there is no way to distinguish to which month the data applies. The first two rows, for instance, represent the quantity and sales amount for Accessories for October and November, but there is no way to tell that from the data. As well, despite the fact that the data represents five months of sales, the columns are named OctQuantity and OctSales. That is because UNION queries get their column names from the names of the columns in the first SELECT statement.

Listing 3.10 shows a query that remedies both of those issues.

Listing 3.10 Tidying up the `UNION` query used to normalize the data

SELECT Category, 'Oct' AS SalesMonth, OctQuantity AS Quantity,
  OctSales AS SalesAmt
FROM SalesSummary
UNION
SELECT Category, 'Nov', NovQuantity, NovSales
FROM SalesSummary
UNION
SELECT Category, 'Dec', DecQuantity, DecSales
FROM SalesSummary
UNION
SELECT Category, 'Jan', JanQuantity, JanSales
FROM SalesSummary
UNION
SELECT Category, 'Feb', FebQuantity, FebSales
FROM SalesSummary;

Table 3.3 shows the same partial extract returned by the query in Listing 3.10.

Table 3.3 Partial extract of data returned by the `UNION` query in Listing 3.10

Category	SalesMonth	Quantity	SalesAmount
Accessories	Dec	987	62758.14
Accessories	Feb	979	60242.47
. . .	. . .	. . .	. . .
Bikes	Nov	412	546657.00
Bikes	Oct	413	536590.50
Car racks	Dec	115	20137.05
Car racks	Feb	124	21763.30
Car racks	Jan	142	24794.75
. . .	. . .	. . .	. . .
Skateboards	Nov	203	89040.58
Skateboards	Oct	164	60530.06
Tires	Dec	150	4388.55
Tires	Feb	137	3937.70
Tires	Jan	186	5377.60
Tires	Nov	110	3081.24
Tires	Oct	151	4356.91

Should you want the data presented in a different sequence, the ORDER BY clause must appear after the last SELECT in the UNION query, as shown in Listing 3.11 on the next page.

Listing 3.11 Specifying the sort order of the `UNION` query

SELECT Category, 'Oct' AS SalesMonth, OctQuantity AS Quantity,
  OctSales AS SalesAmt
FROM SalesSummary
UNION
SELECT Category, 'Nov', NovQuantity, NovSales
FROM SalesSummary
UNION
SELECT Category, 'Dec', DecQuantity, DecSales
FROM SalesSummary
UNION
SELECT Category, 'Jan', JanQuantity, JanSales
FROM SalesSummary
UNION
SELECT Category, 'Feb', FebQuantity, FebSales
FROM SalesSummary
ORDER BY SalesMonth, Category;

Table 3.4 shows a partial extract returned by the query in Listing 3.11.

Table 3.4 Partial extract of data returned by the `UNION` query in Listing 3.11

Category	SalesMonth	Quantity	SalesAmount
Accessories	Dec	987	62758.14
Bikes	Dec	332	439831.50
Car racks	Dec	115	20137.05
Clothing	Dec	139	4937.74
Components	Dec	265	27480.22
Skateboards	Dec	129	59377.20
Tires	Dec	150	4388.55
Accessories	Feb	979	60242.47
Bikes	Feb	450	585130.50
Car racks	Feb	124	21763.30
. . .	. . .	. . .	. . .

NOTE

Some DBMSs (such as Microsoft Access) allow you to put ORDER BY clauses other than at the end, but they do not actually cause the order to change.

When specifying the columns in the ORDER BY clause, usually you have the option of referring to them by name (remembering that the column names are specified in the first SELECT) or by position number. In other words, Listing 3.11 could use ORDER BY 2, 1 instead of ORDER BY SalesMonth, Category. Oracle, however, insists on using ordinal references.

Another consideration is that UNION queries eliminate any duplicate rows. Should this not be what you want, you can specify UNION ALL instead of UNION, and duplicates will not be eliminated. On the other hand, UNION ALL can provide performance improvements as it skips the step of deduplicating the result set, so if you know that the sources will not overlap, it can be advantageous to specify UNION ALL for those queries.

Things to Remember

Each of the SELECT statements in the UNION query must have the same number of columns.
Although the names of the columns in the various SELECT statements do not matter, the data types of each column must be compatible.
To control the order in which the data appears, you can use an ORDER BY clause after the last SELECT statement.
Use UNION ALL rather than UNION if you do not wish to eliminate duplicate rows or pay the performance penalty of deduplicating rows.

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