I stumbled upon an interesting question on Stack Overflow recently. A user wanted to query a table for a given predicate. If that predicate returns no rows, they wanted to run another query using a different predicate. Preferably in a single query. Challenge accepted! Canonical Idea: Use a Common Table Expression We're querying the Sakila … Continue reading How to Execute a SQL Query Only if Another SQL Query has no Results
Category: sql
This category contains posts about SQL in general or some RDBMS’s SQL dialect
When to Use Bind Values, and When to Use Inline Values in SQL
Users of jOOQ, PL/SQL, T-SQL are spoiled as they hardly ever need to worry about bind values. Consider the following statements: Using jOOQ public int countActors(String firstName, String lastName) { return ctx.selectCount() .from(ACTOR) .where(ACTOR.FIRST_NAME.eq(firstName)) .and(ACTOR.LAST_NAME.eq(lastName)) .fetchOneInto(int.class); ); } The method parameters firstName and lastName will be automatically mapped to bind values in the generated SQL … Continue reading When to Use Bind Values, and When to Use Inline Values in SQL
How to Generate at Least One Row in SQL
There are some situations where you would like to have at least one (empty) row in your result set in SQL. Imagine the following situation. We're querying the Sakila database for actors and their films: SELECT first_name, last_name, title FROM actor JOIN film_actor USING (actor_id) JOIN film USING (film_id) ORDER BY 1, 2, 3 yielding … Continue reading How to Generate at Least One Row in SQL
The Difficulty of Tuning Queries Over a Database Link – Or How I Learned to Stop Worrying and Love the DUAL@LINK Table
A large-ish customer in banking (largest tables on that particular system: ~1 billion rows) once decided to separate the OLTP database from the "log database" in order to better use resources and prevent contention on some tables, as the append-only log database is used heavily for analytic querying of all sorts. That seems to make … Continue reading The Difficulty of Tuning Queries Over a Database Link – Or How I Learned to Stop Worrying and Love the DUAL@LINK Table
Don’t Use the String Concatenation “Trick” in SQL Predicates
In SQL, quite often, we want to compare several values with each other. For instance, when we're looking for a specific user by their first and last names, we'll write a query like this one: SELECT * FROM customer WHERE first_name = 'SUSAN' AND last_name = 'WILSON'; We're getting: CUSTOMER_ID FIRST_NAME LAST_NAME ------------------------------------ 8 SUSAN … Continue reading Don’t Use the String Concatenation “Trick” in SQL Predicates
How to Calculate Multiple Aggregate Functions in a Single Query
At a customer site, I've recently encountered a report where a programmer needed to count quite a bit of stuff from a single table. The counts all differed in the way they used specific predicates. The report looked roughly like this (as always, I'm using the Sakila database for illustration): -- Total number of films … Continue reading How to Calculate Multiple Aggregate Functions in a Single Query
How to Fetch Multiple Oracle Execution Plans in One Nice Query
When looking at execution plans in Oracle, we'll have to do several steps to be able to call the DBMS_XPLAN package functions. In fact, we have to find out the SQL_ID for a given statement first, and only then we can get its plan. I've blogged about this previously, here. However, thanks to lateral unnesting, … Continue reading How to Fetch Multiple Oracle Execution Plans in One Nice Query
SQL IN Predicate: With IN List or With Array? Which is Faster?
Hah! Got nerd-sniped again: https://stackoverflow.com/questions/43099226/how-to-make-jooq-to-use-arrays-in-the-in-clause/43102102 A jOOQ user was wondering why jOOQ would generate an IN list for a predicate like this: Java COLUMN.in(1, 2, 3, 4) SQL COLUMN in (?, ?, ?, ?) ... when in fact there could have been the following predicate being generated, instead: COLUMN = any(?::int[]) In the second case, … Continue reading SQL IN Predicate: With IN List or With Array? Which is Faster?
How to Benchmark Alternative SQL Queries to Find the Fastest Query
Tuning SQL isn't always easy, and it takes a lot of practice to recognise how any given query can be optimised. One of the most important slides of my SQL training is the one summarising "how to be fast": Some of these bullets were already covered on this blog. For instance avoiding needless, mandatory work, … Continue reading How to Benchmark Alternative SQL Queries to Find the Fastest Query
Creating Tables Dum and Dee in PostgreSQL
I was nerd-sniped: https://twitter.com/benjiweber/status/842725600280428545 So tables dee and dum are two theoretical tables in SQL, and they can be characterised as such: [Dee] is the relation that has no attributes and a single tuple. It plays the role of True. [Dum] is the relation that has no attributes and no tuples. It plays the role … Continue reading Creating Tables Dum and Dee in PostgreSQL