Comparison query

Question

I have a table called vehicle_density. Following are the sample data of this table.
Period          Vehicle       Location
Jan-2009     300     Jayanagar
Fab-2009     245     Jayanagar
Mar-2009     236     Jayanagar
Apr-2009     298     Jayanagar
May-2009     325     Jayanagar
Jun-2009     204     Jayanagar
Jan-2009     568     BTM
Fab-2009     585     BTM
Mar-2009     401     BTM
Apr-2009     565     BTM
May-2009     621     BTM
Jun-2009     425     BTM
Jan-2009     145     RT Nagar
Fab-2009     200     RT Nagar
Mar-2009     254     RT Nagar
Apr-2009     120     RT Nagar
May-2009     282     RT Nagar
Jun-2009     96     RT NagarNow I need a query to compare vehicle density between different locations for given time period.
For example here is sample output vehicle density comparison between BTM and Jayanagar between Jan-2009 to Mar-2009
Period       Jayanagar   BTM
Jan-2009     300     568
Fab-2009     245     585
Mar-2009     236     401Same way density comparison between jayanagar, BTM and RT Nagar between Jan-2009 to Mar-2009 is below
Period     Jayanagar     BTM     RT Nagar
Jan-2009     300     568     145
Fab-2009     245     585     200
Mar-2009     236     401     254
Apr-2009     298     565     120Can I do it in single sql query?
Thanks,
Sujnan

Answer 1

Hi, Sujnan,
Yes, you can do that in one query. It's called a pivot.
One way is shown below. This example uses the COUNT function; for what you want, use SUM instead.
Search for "pivot" or "rows to columns" for more examples.
--     How to Pivot a Result Set (Display Rows as Columns)
--     For Oracle 10, and earlier
--     Actually, this works in any version of Oracle, but the
--     "SELECT ... PIVOT" feature introduced in Oracle 11
--     is better.  (See Query 2, below.)
--     This example uses the scott.emp table.
--     Given a query that produces three rows for every department,
--     how can we show the same data in a query that has one row
--     per department, and three separate columns?
--     For example, the query below counts the number of employess
--     in each departent that have one of three given jobs:
PROMPT     ==========  0. Simple COUNT ... GROUP BY  ==========
SELECT     deptno
,     job
,     COUNT (*)     AS cnt
FROM     scott.emp
WHERE     job     IN ('ANALYST', 'CLERK', 'MANAGER')
GROUP BY     deptno
,          job;
Output:
    DEPTNO JOB              CNT
        20 CLERK              2
        20 MANAGER            1
        30 CLERK              1
        30 MANAGER            1
        10 CLERK              1
        10 MANAGER            1
        20 ANALYST            2
PROMPT     ==========  1. Pivot  ==========
SELECT     deptno
,     COUNT (CASE WHEN job = 'ANALYST' THEN 1 END)     AS analyst_cnt
,     COUNT (CASE WHEN job = 'CLERK'   THEN 1 END)     AS clerk_cnt
,     COUNT (CASE WHEN job = 'MANAGER' THEN 1 END)     AS manager_cnt
FROM     scott.emp
WHERE     job     IN ('ANALYST', 'CLERK', 'MANAGER')
GROUP BY     deptno;
--     Output:
    DEPTNO ANALYST_CNT  CLERK_CNT MANAGER_CNT
        30           0          1           1
        20           2          2           1
        10           0          1           1
--     Explanation
(1) Decide what you want the output to look like.
     (E.g. "I want a row for each department,
     and columns for deptno, analyst_cnt, clerk_cnt and manager_cnt)
(2) Get a result set where every row identifies which row
     and which column of the output will be affected.
     In the example above, deptno identifies the row, and
     job identifies the column.
     Both deptno and job happened to be in the original table.
     That is not always the case; sometimes you have to
     compute new columns based on the original data.
(3) Use aggregate functions and CASE (or DECODE) to produce
     the pivoted columns. 
     The CASE statement will pick
     only the rows of raw data that belong in the column.
     If each cell in the output corresponds to (at most)
     one row of input, then you can use MIN or MAX as the
     aggregate function.
     If many rows of input can be reflected in a single cell
     of output, then use SUM, COUNT, AVG, STRAGG, or some other
     aggregate function.
     GROUP BY the column that identifies rows.
PROMPT     ==========  2. Oracle 11 PIVOT  ==========
WITH     e     AS
(     -- Begin sub-query e to SELECT columns for PIVOT
     SELECT     deptno
     ,     job
     FROM     scott.emp
)     -- End sub-query e to SELECT columns for PIVOT
SELECT     *
FROM     e
PIVOT     (     COUNT (*)
          FOR     job     IN     ( 'ANALYST'     AS analyst
                         , 'CLERK'     AS clerk
                         , 'MANAGER'     AS manager
NOTES ON ORACLE 11 PIVOT:
(1) You must use a sub-query to select the raw columns.
An in-line view (not shown) is an example of a sub-query.
(2) GROUP BY is implied for all columns not in the PIVOT clause.
(3) Column aliases are optional. 
If "AS analyst" is omitted above, the column will be called 'ANALYST' (single-quotes included).
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