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Showing entries 1 to 10 of 23 10 Older Entries

Displaying posts with tag: syntax (reset)

Merging tables with INSERT...ON DUPLICATE KEY UPDATE
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Had a case recently where I had to merge data from two identically structured tables containing nearly identical data.

"Nearly identical" meaning most table data is identical in both; sometimes a row is missing from one of the tables; sometimes same row (according to PK) appears in both, but some columns are NULL is one tables (while others can be NULL in the second).

Otherwise no contradicting data: it was not possible for some data to be "3" in one table and "4" in the other.

How do you create a merge of the tables, such that all missing rows are completed, and NULLs replaced by actual values when possible?

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Pop quiz: funny syntax
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The following questions are of little importance, yet I find them entertaining. I stumbled upon these while developing QueryScript.

Can you guess the results of the following statements?

Pop quiz 1

SET @x := 7;
SELECT ++@x;

What is the computation result? What will be the value of @x?

Pop quiz 2

SET @ := 4;
SELECT @ + @'' + @``

What is the computation result?

Pop quiz 3

SET @a := 2;
SELECT @a = @'a' = @`a`

Do we get TRUE or FALSE? When?

Hints

Consider the following queries as hints to the above …

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Using CURDATE() the wrong way
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It has been a while since I wrote on this blog. Basically I had too much on my mind (expanding my department, holidays, etc) to actually write here and I’ll promise to post more regularly from now onwards. ;)

Anyway, as the title already suggests: I found out how you can use CURDATE() in a wrong way. One of the developers in my company asked me to help him out as his code all of a sudden did not work properly anymore. Or even better: it used to process several thousands of rows and all of a sudden it processed none.

I looked at his …

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MySQL joins: ON vs. USING vs. Theta-style
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What is the difference between the following three syntaxes?

SELECT * FROM film JOIN film_actor ON (film.film_id = film_actor.film_id)
SELECT * FROM film JOIN film_actor USING (film_id)
SELECT * FROM film, film_actor WHERE film.film_id = film_actor.film_id

The difference is mostly syntactic sugar, but with a couple interesting notes.

To put names, the first two are called "ANSI-style" while the third is called "Theta-style".

Theta style

On the FROM clause, tables are listed as if with Cartesian products, and the WHERE clause specifies how the …

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Syntax of the day: IS TRUE and IS FALSE
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What makes for a true statement?

We usually test statements using a WHERE clause:

SELECT * FROM world.City WHERE Population > 1000000

The "Population > 1000000" statement makes for a boolean expression. Using WHERE is just one way of evaluating it. One can also test with IF():

SET @val := 7;
SELECT IF(@val > 2, 'Yes', 'No')

TRUE and FALSE

The two are keywords. They also map for the numerals 1 and …

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Quoting text JavaScript/Python style
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Unless your MySQL is configured to use ANSI_QUOTES in sql_mode, you are able to quote your text in one of two forms: using single quotes or double quotes:

UPDATE world.Country SET HeadOfState = 'Willy Wonka' WHERE Code='USA'
UPDATE world.Country SET HeadOfState = "Willy Wonka" WHERE Code="USA"

This makes for JavaScript- or Python-style quoting: you quote by your needs. Say you have a text which includes single quotes:

It is …

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MySQL terminology: processes, threads & connections
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There’s some confusion in the MySQL terminology for processes, threads & connections, which I will try to resolve. I can’t explain the full nature of what processes and threads are; please see Wikipedia [1] [2] for that. But here’s some basics with regard to MySQL:

  • MySQL server is a single process application.
  • It is multithreaded.
  • It (usually) acts as a TCP/IP server, accepting connections.
  • Each connection gets a dedicated …
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SQL: good comments conventions
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I happened upon a customer who left me in awe and admiration. The reason: excellent comments for their SQL code.

I list four major places where SQL comments are helpful. I’ll use the sakila database. It is originally scarcely commented; I’ll present it now enhanced with comments, to illustrate.

Table definitions

The CREATE TABLE statement allows for a comment, intended to describe the nature of the table:

CREATE TABLE `film_text` (
 `film_id` smallint(6) NOT NULL,
 `title` varchar(255) NOT NULL,
 `description` text,
 PRIMARY KEY …
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MySQL Idiosyncrasies That Bite
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The following are my slides that I presented at ODTUG Kaleidoscope 2010. This presentation talks about the MySQL defaults including a non-transactional state, silent data truncations, date management, and transaction isolation options. These are all critical for data integrity and consistency. I also cover in-depth topics including SQL_MODE, character sets and collations.

MySQL Idiosyncrasies That Bite View more …

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Views: better performance with condition pushdown
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Justin’s A workaround for the performance problems of TEMPTABLE views post on mysqlperformanceblog.com reminded me of a solution I once saw on a customer’s site.

The customer was using nested views structure, up to depth of some 8-9 views. There were a lot of aggregations along the way, and even the simplest query resulted with a LOT of subqueries, temporary tables, and vast amounts of data, even if only to return with a couple …

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Showing entries 1 to 10 of 23 10 Older Entries

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