ARRAY types are a part of the ISO/IEC 9075 SQL standard. The standard specifies how to: Construct arrays Nest data into arrays (e.g. by means of aggregation or subqueries) Unnest data from arrays into tables But it is very unopinionated when it comes to function support. The ISO/IEC 9075-2:2023(E) 6.47 <array value expression> specifies concatenation … Continue reading When SQL Meets Lambda Expressions
Category: sql
This category contains posts about SQL in general or some RDBMS’s SQL dialect
Think About SQL MERGE in Terms of a RIGHT JOIN
RIGHT JOIN is an esoteric feature in the SQL language, and hardly ever seen in the real world, because almost every RIGHT JOIN can just be expressed as an equivalent LEFT JOIN. The following two statements are equivalent: -- Popular SELECT c.first_name, c.last_name, p.amount FROM customer AS c LEFT JOIN payment AS p ON c.customer_id … Continue reading Think About SQL MERGE in Terms of a RIGHT JOIN
Emulating SQL FILTER with Oracle JSON Aggregate Functions
A cool standard SQL:2003 feature is the aggregate FILTER clause, which is supported natively by at least these RDBMS: ClickHouse CockroachDB DuckDB Firebird H2 HSQLDB PostgreSQL SQLite Trino YugabyteDB The following aggregate function computes the number of rows per group which satifsy the FILTER clause: SELECT COUNT(*) FILTER (WHERE BOOK.TITLE LIKE 'A%'), COUNT(*) FILTER (WHERE … Continue reading Emulating SQL FILTER with Oracle JSON Aggregate Functions
Getting Top 1 Values Per Group in Oracle
I've blogged about generic ways of getting top 1 or top n per category queries before on this blog. An Oracle specific version in that post used the arcane KEEP syntax: SELECT max(actor_id) KEEP (DENSE_RANK FIRST ORDER BY c DESC, actor_id), max(first_name) KEEP (DENSE_RANK FIRST ORDER BY c DESC, actor_id), max(last_name) KEEP (DENSE_RANK FIRST ORDER … Continue reading Getting Top 1 Values Per Group in Oracle
An Efficient Way to Check for Existence of Multiple Values in SQL
In a previous blog post, we've advertised the use of SQL EXISTS rather than COUNT(*) to check for existence of a value in SQL. I.e. to check if in the Sakila database, actors called WAHLBERG have played in any films, instead of: SELECT count(*) FROM actor a JOIN film_actor fa USING (actor_id) WHERE a.last_name = … Continue reading An Efficient Way to Check for Existence of Multiple Values in SQL
Workaround for MySQL’s “can’t specify target table for update in FROM clause” Error
In MySQL, you cannot do this: create table t (i int primary key, j int); insert into t values (1, 1); update t set j = (select max(j) from t) + 1; The UPDATE statement will raise an error as follows: SQL Error [1093] [HY000]: You can't specify target table 't' for update in FROM … Continue reading Workaround for MySQL’s “can’t specify target table for update in FROM clause” Error
Maven Coordinates of the most popular JDBC Drivers
Do you need to add a JDBC driver to your application, and don't know its Maven coordinates? This blog post lists the most popular drivers from the jOOQ integration tests. Look up the latest versions directly on https://central.sonatype.com/ with parameters g:groupId a:artifactId, for example, the H2 database and driver: https://central.sonatype.com/search?q=g%3Acom.h2database+a%3Ah2 The list only includes drivers … Continue reading Maven Coordinates of the most popular JDBC Drivers
JDBC Connection URLs of the Most Popular RDBMS
Need to connect to your RDBMS with JDBC and don't have the JDBC connection URL or driver name at hand? No problem, just look up your RDBMS below: // BigQuery driver = "com.simba.googlebigquery.jdbc42.Driver"; url = "jdbc:bigquery://https://www.googleapis.com/bigquery/v2:443;ProjectId=<project-id>;OAuthType=0;OAuthServiceAcctEmail=<service-account>;OAuthPvtKeyPath=path-to-key.json"; // CockroachDB driver = "org.postgresql.Driver"; url = "jdbc:postgresql://<host>/<database>"; // Db2 driver = "com.ibm.db2.jcc.DB2Driver"; url = "jdbc:db2://<host>:50000/<database>"; // Derby driver … Continue reading JDBC Connection URLs of the Most Popular RDBMS
How to Turn a List of Flat Elements into a Hierarchy in Java, SQL, or jOOQ
Occasionally, you want to write a SQL query and fetch a hierarchy of data, whose flat representation may look like this: SELECT id, parent_id, label FROM t_directory; The result might be: |id |parent_id|label | |---|---------|-------------------| |1 | |C: | |2 |1 |eclipse | |3 |2 |configuration | |4 |2 |dropins | |5 |2 |features | … Continue reading How to Turn a List of Flat Elements into a Hierarchy in Java, SQL, or jOOQ
How to Write a Derived Table in jOOQ
One of the more frequent questions about jOOQ is how to write a derived table (or a CTE). The jOOQ manual shows a simple example of a derived table: In SQL: SELECT nested.* FROM ( SELECT AUTHOR_ID, count(*) books FROM BOOK GROUP BY AUTHOR_ID ) nested ORDER BY nested.books DESC In jOOQ: // Declare the … Continue reading How to Write a Derived Table in jOOQ