Tag: Syntax

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?

pt-table-sync comes to mind: one can do a bidirectional syncing of two tables, and actually stating how to resolve ambiguities (like “greater value wins”). Very powerful! An example would be:

pt-table-sync --bidirectional --conflict-column=a --conflict-comparison=greatest --tables ...

However I didn’t actually have any problem with the tables themselves. The two tables were just fine as they were; missing or NULL data does not indicate an error on their part. I wanted to get their merge. pt-table-sync is still up for the job: we can duplicate them, merge on the copy… But I prefer a query over an external script when possible.

INSERT…ON DUPLICATE KEY UPDATE

This MySQL-specific syntax is actually quite powerful. It basically says “if the insert fails due to unique constraint, you get a chance to update the row causing the failure”. But it also allows for smart setting of the column via the VALUES() clause. Let’s present some sample data and then see the solution. Continue reading » “Merging tables with INSERT…ON DUPLICATE KEY UPDATE”

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? Continue reading » “Pop quiz: funny syntax”

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 join should take place.

This is considered to be the “old” style. It is somewhat confusing to read. Consider the following query: Continue reading » “MySQL joins: ON vs. USING vs. Theta-style”

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 0, as follows:

mysql> SELECT TRUE, FALSE;
+------+-------+
| TRUE | FALSE |
+------+-------+
|    1 |     0 |
+------+-------+

Like in the C programming language, a nonzero value evaluates to a true value. A zero evaluates to false. A NULL evaluates to… well, NULL. But aside from 3-valued logic, what’s important in our case is that it is not true. Continue reading » “Syntax of the day: IS TRUE and IS FALSE”

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 what you read when you don’t have to that determines what you will be when you can’t help it. – Oscar Wilde

You wish to insert this text to some tables. You could go through the trouble of escaping it:

INSERT INTO quotes (quote, author) VALUES (
  'It is what you read when you don\'t have to that determines what you will be when you can\'t help it.', 'Oscar Wilde');

or you could just wrap it in double quotes:

INSERT INTO quotes (quote, author) VALUES (
  "It is what you read when you don't have to that determines what you will be when you can't help it.", 'Oscar Wilde');

I find this useful when using SQL to generate queries. Take, for example, eval() for MySQL: the statement: Continue reading » “Quoting text JavaScript/Python style”

I happened on a query where, by mistake, an

SELECT ... ORDER BY x DESC LIMIT 1

was written as

SELECT ... GROUP BY x DESC LIMIT 1

And it took me by surprise to realize GROUP BY x DESC is a valid statement. I looked it up: yep! It’s documented.

In MySQL, GROUP BY results are sorted according to the group statement. You can override this by adding ORDER BY NULL (see past post). I wasn’t aware you can actually control the sort order.