Use CASE and GROUP BY to dynamically replace PIVOT
Question:
The data shown in the table below is organized in rows and columns. The goal is to convert this into a table with a dynamic number of columns, where each column represents a value grouped by a specified category.
id | feh | bar |
---|---|---|
1 | 10 | A |
2 | 20 | A |
3 | 3 | B |
4 | 4 | B |
5 | 5 | C |
6 | 6 | D |
7 | 7 | D |
8 | 8 | D |
Expected output:
bar | val1 | val2 | val3 |
---|---|---|---|
A | 10 | 20 | |
B | 3 | 4 | |
C | 5 | ||
D | 6 | 7 | 8 |
Original query:
The following query uses CASE expressions and GROUP BY to achieve the desired results:
SELECT bar, MAX(CASE WHEN abc."row" = 1 THEN feh ELSE NULL END) AS "val1", MAX(CASE WHEN abc."row" = 2 THEN feh ELSE NULL END) AS "val2", MAX(CASE WHEN abc."row" = 3 THEN feh ELSE NULL END) AS "val3" FROM ( SELECT bar, feh, row_number() OVER (partition by bar) as row FROM "Foo" ) abc GROUP BY bar
Efficient crosstab alternative:
To improve efficiency and readability, you can use the crosstab function in the tablefunc module to implement a dynamic solution. Here's an example:
SELECT * FROM crosstab( 'SELECT bar, 1 AS cat, feh FROM tbl_org ORDER BY bar, feh') AS ct (bar text, val1 int, val2 int, val3 int); -- 更多列?
Handling multiple values:
For scenarios where there are multiple values under the same category, the crosstab function can be extended to the following form:
SELECT * FROM crosstab( 'SELECT bar, val, feh FROM tbl_org ORDER BY 1, 2') AS ct (bar text, val1 int, val2 int, val3 int); -- 更多列?
Built-in crosstab function:
The tablefunc module also provides predefined crosstab functions for a specific number of columns:
SELECT * FROM crosstab3('SELECT row_name, attrib, val FROM tbl ORDER BY 1,2');
These functions make calling simpler and handle text data by default.
Dynamic return type:
Although tablefunc simplifies the process, it has limitations in handling dynamic return types. To solve this problem, other methods can be considered, such as using PL/pgSQL functions or creating dynamic SQL statements.
The above is the detailed content of How to Efficiently Perform a Crosstab Query in SQL?. For more information, please follow other related articles on the PHP Chinese website!

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