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Displaying posts with tag: null (reset)
Treating NULLs as not less than zero in ORDER BY Revisited

In my post yesterday, I shared a little known trick for sorting NULLs last when using ORDER BY ASC.

To summarize briefly, NULLs are treated as less than 0 when used in ORDER BY, However, sometimes you do not want that behavior, and you need the NULLs listed last, even though you want your numbers in ascending order.

So a query like the following returns the NULLs first (expected behavior):

| col1   | col2 |
| apple  | NULL |
| apple  |    5 |
| apple  |   10 |
| banana | NULL |
| banana |    5 |
| banana |   10 |

The trick I mentioned in my post is to rewrite the query like:


The difference is that we added a minus sign (-) in front of the column …

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Treating NULLs as not less than zero in ORDER BY

I was working on a seemingly basic query the other day where the user needed to have an INT column listed in ascending order (i.e., 1, 2, 3, …).

However, the tricky part came in because the column allowed NULLs and the user needed the NULLs to be listed last, not first, which is the default behavior in both MariaDB and MySQL.

We first devised a somewhat convoluted solution where we used ISNULL() first in the ORDER BY, and then the column, but that wasn’t ideal since it added an additional check for each row in the ORDER BY, which we wanted to avoid in a query returning ~5M rows.

To illustrate, a normal query just sorting in ASC order returned:

MariaDB> SELECT * FROM t1 ORDER BY col1 ASC, col3 ASC;
| col1   | col2   | col3 |
| apple  | yellow | NULL |
| apple  | red    |    5 |
| apple  | green  |   10 |
| banana | brown  | NULL |
| banana | green  |    5 |
| …
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NOT NULL all the things!

Different types of languages deal with this “value” in diverse ways. You can have a more comprehensive list of what NULL can mean on this website. What I like to think about NULL is along the lines of invalid, as if some sort of garbage is stored there. It doesn’t mean it’s empty, it’s just mean that something is there, and it has no value to you.

Databases deal when storing this type in a similar way, PostgreSQL treats it as “unknown” while MySQL treats it as “no data“.

Both databases recommend using \N to represent NULL values where import or exporting of data is necessary.

When …

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10 Common Mistakes Java Developers Make when Writing SQL

This article is part of a series. You might also like: 10 More Common Mistakes Java Developers Make when Writing SQLYet Another 10 Common Mistakes Java Developers Make When Writing SQL Java developers mix object-oriented thinking with imperative thinking, depending on their levels of: Skill (anyone can code imperatively)Dogma (some use the "Pattern-Pattern", i.e. the … Continue reading 10 Common Mistakes Java Developers Make when Writing SQL →

10 Things in SQL Server Which Don’t Work as Expected

So far, I have been blogging about curious RDBMS caveats mostly related to Oracle and MySQL databases. Some examples: You never stop learning about Oracle features NOT IN vs. NOT EXISTS vs. LEFT JOIN / IS NULL: MySQL SQL incompatibilities: NOT IN and NULL values MySQL Bad Idea #384 But there are also other databases, … Continue reading 10 Things in SQL Server Which Don’t Work as Expected →

Bulk insert into tables in sorted order to avoid deadlocks

Shard-Query inserts data into a “coordinator” table when answering queries.   When there is a GROUP BY on the original query, the coordinator table contains a UNIQUE KEY over the GROUP BY attributes.   Shard-Query uses INSERT .. ON DUPLICATE KEY UPDATE in combination with bulk insert (insert into … values (),(),() ) when inserting into the table.

For what would normally be efficiency sake, Shard-Query sends queries to the shards using ORDER BY NULL which disables the filesort operation. Of course, this often results in the rows being sent back from the shards in random order.

Because the results are in random order, the bulk insertion that the worker does into the coordinator table can deadlock with other worker threads when using InnoDB or TokuDB as the coordinator table. Right now I’ve just been using MyISAM for the coordinator table, which serializes queries at the bulk insert stage.  Having to insert the …

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Two subtle bugs in OUTER JOIN queries

OUTER JOIN queries in SQL are susceptible to two very subtle bugs that I’ve observed a number of times in the real world. Daniel and I have been hammering out ways to automatically detect queries that suffer from these bugs, in a relatively new Maatkit tool called mk-query-advisor. It’s part of our series of advisor tools for MySQL. I wrote a blog post about it a while ago. Automated analysis of bad query patterns is a good thing to write tools to do, because catching buggy queries is hard work if you do it manually.

Let’s dive right in and analyze these subtle bugs. Warning: if you don’t understand how SQL handles NULL, you’re not going to understand the following. Many people have a hard time with NULL, which is why these bugs are so hard to …

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The difference between a unique index and primary key in MySQL

There’s a really important difference between a unique index (MySQL’s answer to a “unique constraint”) and a primary key in MySQL. Please take a look at this:

  `a` int,
  `b` int,
  `c` int,
  UNIQUE KEY `a` (`a`,`b`)

The combination of columns a, b should uniquely identify any tuple in the table, right?

select * from t;
| a    | b    | c    |
|    1 |    2 |    3 | 
| NULL | NULL |    1 | 
| NULL | NULL |    1 | 
| NULL | NULL |    1 | 

Wrong. Our arch-enemy NULL messes things up again:

A UNIQUE index creates a constraint such that all values in the index must be distinct. An error occurs if you try to add a new row with a key value that matches an existing row. This constraint does …

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MySQL – NULL vs ” vs ‘NULL’

Today, in one of my project I’ve to check empty fields.

The field name is: answer it’s type is TEXT and Default value is NULL

In my SQL query I was checked at first answer != NULL (if the answer is not null, i.e if the answer is not empty), But this was not showing the correct result.

Then I changed it to answer != ” ( i.e ” means empty string) then it showed the correct result.
Then I test with this answer != ‘NULL’, and it also showed the correct result.

Calculating the Nth percentile in MySQL

Yesterday, I was on the freenode ##pentaho irc channel when Andres Chaves asked me how to calculate the Nth percentile in MySQL. He saw a solution somewhere using subqueries, but wasn't too happy about it.

A while ago I wrote about calulating the median in MySQL, and it turns out the Nth percentile can be calculated using a similar, single-pass approach, not relying on subqueries, UDFs, or user-defined variables.
The percentile....
So, what is a percentile exactly? Here's what the wikipedia says:

A percentile is the value of a variable below which a certain percent of observations fall. So the 20th percentile is the value (or score) below which 20 …

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