[QA} Test ORDER BY/LIMIT Combinations
Introduction
In this article, we will be testing the ORDER BY/LIMIT combinations in a query. We will be varying several parameters to see how they affect the results. The query we will be using is of the following form:
SELECT field FROM table ORDER BY field1 [DESC|ASC] [NULLS FIRST|NULLS LAST] LIMIT N
We will be testing the following variations:
- Number of identical elements in the collection
- DESC/ASC
- NULLS FIRST/NULLS LAST/UNSPECIFIED
- LIMIT VALUE (1, 2, 3)
Testing Environment
For this test, we will be using a database with a table containing the following data:
| field1 | field2 |
|---|---|
| 1 | a |
| 2 | b |
| 3 | c |
| 4 | d |
| 5 | e |
| 6 | f |
| 7 | g |
| 8 | h |
| 9 | i |
| 10 | j |
| 11 | k |
| 12 | l |
| 13 | m |
| 14 | n |
| 15 | o |
| 16 | p |
| 17 | q |
| 18 | r |
| 19 | s |
| 20 | t |
Testing NULLS FIRST/NULLS LAST/UNSPECIFIED
In this test, we will be varying the NULLS FIRST/NULLS LAST/UNSPECIFIED parameter.
NULLS FIRST
When we use NULLS FIRST, the NULL values will be placed first in the sorted list.
SELECT field1 FROM table ORDER BY field1 NULLS FIRST LIMIT 5
The results of this query will be:
| field1 |
|---|
| NULL |
| 1 |
| 2 |
| 3 |
| 4 |
NULLS LAST
When we use NULLS LAST, the NULL values will be placed last in the sorted list.
SELECT field1 FROM table ORDER BY field1 NULLS LAST LIMIT 5
The results of this query will be:
| field1 |
|---|
| 1 |
| 2 |
| 3 |
| 4 |
| 5 |
UNSPECIFIED
When we do not specify NULLS FIRST or NULLS LAST, the NULL values will be placed last in the sorted list.
SELECT field1 FROM table ORDER BY field1 LIMIT 5
The results of this query will be:
| field1 |
|---|
| 1 |
| 2 |
| 3 |
| 4 |
| 5 |
Testing DESC/ASC
In this test, we will be varying the DESC/ASC parameter.
DESC
When we use DESC, the values will be sorted in descending order.
SELECT field1 FROM table ORDER BY field1 DESC LIMIT 5
The results of this query will be:
| field1 |
|---|
| 20 |
| 19 |
| 18 |
| 17 |
| 16 |
ASC
When we use ASC, the values will be sorted in ascending order.
SELECT field1 FROM table ORDER BY field1 ASC LIMIT 5
The results of this query will be:
| field1 |
|---|
| 1 |
| 2 |
| 3 |
| 4 |
| 5 |
Testing LIMIT VALUE
In this test, we will be varying the LIMIT VALUE parameter.
LIMIT 1
When we use LIMIT 1, only one value will be returned.
SELECT field1 FROM table ORDER BY field1 DESC LIMIT 1
The results of this query will be:
| field1 |
|---|
| 20 |
LIMIT 2
When we use LIMIT 2, two values will be returned.
SELECT field1 FROM table ORDER BY field1 DESC LIMIT 2
The results of this query will be:
| field1 |
|---|
| 20 |
| 19 |
LIMIT 3
When we use LIMIT 3, three values will be returned.
SELECT field1 FROM table ORDER BY field1 DESC LIMIT 3
The results of this query will be:
| field1 |
|---|
| 20 |
| 19 |
| 18 |
Comparison with MySQL
MySQL has a different default comparison for NULL values. In MySQL, NULL values are considered greater than all other values. This means that when we use ORDER BY without specifying NULLS FIRST or NULLS LAST, the NULL values will be placed last in the sorted list.
SELECT field1 FROM table ORDER BY field1 LIMIT 5
The results of this query will be:
| field1 |
|---|
| 1 |
| 2 |
| 3 |
| 4 |
| 5 |
However, when we use ORDER BY with DESC, the NULL values will be placed first in the sorted list.
SELECT field1 FROM table ORDER BY field1 DESC LIMIT 5
The results of this query will be:
| field1 |
|---|
| NULL |
| 20 |
| 19 |
| 18 |
| 17 |
Conclusion
Q&A
Q: What is the purpose of the ORDER BY clause in a query? A: The ORDER BY clause is used to sort the result set of a query in ascending or descending order based on one or more columns.
Q: What is the difference between ASC and DESC in the ORDER BY clause? A: ASC (ascending) sorts the result set in ascending order, while DESC (descending) sorts the result set in descending order.
Q: What is the purpose of the LIMIT clause in a query? A: The LIMIT clause is used to limit the number of rows returned in the result set.
Q: How does the NULLS FIRST and NULLS LAST clauses work in the ORDER BY clause? A: The NULLS FIRST clause places NULL values first in the sorted list, while the NULLS LAST clause places NULL values last in the sorted list.
Q: What is the default behavior of the ORDER BY clause when NULL values are present? A: The default behavior of the ORDER BY clause when NULL values are present depends on the database management system being used. In some systems, NULL values are considered greater than all other values, while in others, NULL values are considered less than all other values.
Q: How does the LIMIT clause interact with the ORDER BY clause? A: The LIMIT clause can be used in conjunction with the ORDER BY clause to limit the number of rows returned in the result set after sorting.
Q: What is the difference between using LIMIT and OFFSET in a query? A: LIMIT is used to limit the number of rows returned in the result set, while OFFSET is used to skip a specified number of rows before returning the remaining rows.
Q: Can the ORDER BY clause be used with multiple columns? A: Yes, the ORDER BY clause can be used with multiple columns to sort the result set based on multiple criteria.
Q: How does the ORDER BY clause handle duplicate values? A: The ORDER BY clause can be used to handle duplicate values by specifying the column(s) to sort on and the order of sorting.
Q: Can the ORDER BY clause be used with aggregate functions? A: Yes, the ORDER BY clause can be used with aggregate functions to sort the result set based on the aggregated values.
Q: How does the ORDER BY clause interact with the GROUP BY clause? A: The ORDER BY clause can be used with the GROUP BY clause to sort the result set based on the grouped values.
Q: Can the ORDER BY clause be used with subqueries? A: Yes, the ORDER BY clause can be used with subqueries to sort the result set based on the values returned by the subquery.
Q: How does the ORDER BY clause handle NULL values in subqueries? A: The ORDER BY clause can handle NULL values in subqueries by using the NULLS FIRST or NULLS LAST clause.
Q: Can the ORDER BY clause be used with window functions? A: Yes, the ORDER BY clause can be used with window functions to sort the result set based on the values returned by the window function.
Q: How does the ORDER BY clause interact with the PARTITION BY clause? A: The ORDER BY clause can be used with the PARTITION BY clause to sort the result based on the partitioned values.
Q: Can the ORDER BY clause be used with the RANK() function? A: Yes, the ORDER BY clause can be used with the RANK() function to sort the result set based on the ranked values.
Q: How does the ORDER BY clause handle ties in the RANK() function? A: The ORDER BY clause can handle ties in the RANK() function by using the DENSE_RANK() function.
Q: Can the ORDER BY clause be used with the ROW_NUMBER() function? A: Yes, the ORDER BY clause can be used with the ROW_NUMBER() function to sort the result set based on the row numbers.
Q: How does the ORDER BY clause interact with the LAG() function? A: The ORDER BY clause can be used with the LAG() function to sort the result set based on the lagged values.
Q: Can the ORDER BY clause be used with the LEAD() function? A: Yes, the ORDER BY clause can be used with the LEAD() function to sort the result set based on the lead values.
Q: How does the ORDER BY clause handle NULL values in the LAG() and LEAD() functions? A: The ORDER BY clause can handle NULL values in the LAG() and LEAD() functions by using the NULLS FIRST or NULLS LAST clause.
Q: Can the ORDER BY clause be used with the NTILE() function? A: Yes, the ORDER BY clause can be used with the NTILE() function to sort the result set based on the ntile values.
Q: How does the ORDER BY clause interact with the PERCENT_RANK() function? A: The ORDER BY clause can be used with the PERCENT_RANK() function to sort the result set based on the percent rank values.
Q: Can the ORDER BY clause be used with the CUME_DIST() function? A: Yes, the ORDER BY clause can be used with the CUME_DIST() function to sort the result set based on the cumulative distribution values.
Q: How does the ORDER BY clause handle NULL values in the CUME_DIST() function? A: The ORDER BY clause can handle NULL values in the CUME_DIST() function by using the NULLS FIRST or NULLS LAST clause.
Q: Can the ORDER BY clause be used with the PERCENTILE_CONT() function? A: Yes, the ORDER BY clause can be used with the PERCENTILE_CONT() function to sort the result set based on the percentile continuous values.
Q: How does the ORDER BY clause interact with the PERCENTILE_DISC() function? A: The ORDER BY clause can be used with the PERCENTILE_DISC() function to sort the result set based on the percentile discrete values.
Q: Can the ORDER BY clause be used with the LAG() and LEAD() functions in the same query? A: Yes, the ORDER BY clause can be used with the LAG() and LEAD() functions in the same query to sort the result set based on the lagged and lead values.
Q: How does the ORDER BY clause handle NULL values in the LAG() and LEAD() functions in the same query? A: The ORDER BY clause can handle NULL values in the LAG() and LEAD() functions in the same query by using the NULLS FIRST orS LAST clause.
Q: Can the ORDER BY clause be used with the NTILE() function in the same query? A: Yes, the ORDER BY clause can be used with the NTILE() function in the same query to sort the result set based on the ntile values.
Q: How does the ORDER BY clause interact with the PERCENT_RANK() function in the same query? A: The ORDER BY clause can be used with the PERCENT_RANK() function in the same query to sort the result set based on the percent rank values.
Q: Can the ORDER BY clause be used with the CUME_DIST() function in the same query? A: Yes, the ORDER BY clause can be used with the CUME_DIST() function in the same query to sort the result set based on the cumulative distribution values.
Q: How does the ORDER BY clause handle NULL values in the CUME_DIST() function in the same query? A: The ORDER BY clause can handle NULL values in the CUME_DIST() function in the same query by using the NULLS FIRST or NULLS LAST clause.
Q: Can the ORDER BY clause be used with the PERCENTILE_CONT() function in the same query? A: Yes, the ORDER BY clause can be used with the PERCENTILE_CONT() function in the same query to sort the result set based on the percentile continuous values.
Q: How does the ORDER BY clause interact with the PERCENTILE_DISC() function in the same query? A: The ORDER BY clause can be used with the PERCENTILE_DISC() function in the same query to sort the result set based on the percentile discrete values.
Q: Can the ORDER BY clause be used with the LAG() and LEAD() functions, NTILE() function, PERCENT_RANK() function, CUME_DIST() function, and PERCENTILE_CONT() function in the same query? A: Yes, the ORDER BY clause can be used with the LAG() and LEAD() functions, NTILE() function, PERCENT_RANK() function, CUME_DIST() function, and PERCENTILE_CONT() function in the same query to sort the result set based on the lagged, lead, ntile, percent rank, cumulative distribution, and percentile continuous values.
Q: How does the ORDER BY clause handle NULL values in the LAG() and LEAD() functions, NTILE() function, PERCENT_RANK() function, CUME_DIST() function, and PERCENTILE_CONT() function in the same query? A: The ORDER BY clause can handle NULL values in the LAG() and LEAD() functions, NTILE() function, PERCENT_RANK() function, CUME_DIST() function, and PERCENTILE_CONT() function in the same query by using the NULLS FIRST or NULLS LAST clause.
Conclusion
In this article, we have covered the ORDER BY/LIMIT combinations in a query. We have discussed the purpose of the ORDER BY clause, the difference between ASC and DESC, the purpose of the LIMIT clause, and how the NULLS FIRST and NULLS