DuckDB’s SAMPLE clause is a handy feature that allows us to work with a random subset of our data. This is particularly useful when dealing with large datasets where processing the entire dataset might be time-consuming or unnecessary for exploratory data analysis, testing queries, or creating representative samples.
When we use this clause, we can specify the absolute number of rows to return, or a percentage of rows. We also have an option of sampling method to use.
Continue readingMost RDBMSs provide at least a few ways to return rows that contain uppercase characters. Here are some options available in the major RDBMSs.
Continue readingMost RDBMSs provide at least a few ways to return rows that contain lowercase characters. Here are some options available in the major RDBMSs.
Continue readingIn SQL, the CASE statement evaluates a list of conditions and returns one of multiple possible result expressions.
In some ways, the SQL CASE statement is kind of similar to the IF...ELSE statement in that it allows us to check for a given condition and return a different result depending on the outcome.
In SQL, the columns information schema view, which returns information about columns, is an ISO standard view that is supported by most of the major RDBMSs. You can use this view to get information about a column’s data type.
Most of the major RDBMs also provide other methods for getting column information.
Here are examples of getting the data type of a column in various SQL databases.
Continue readingIf you’re trying to run a multi-line query in SQLcl or SQL*Plus, and you keep getting an error such as “Unknown Command”, but running it in SQL Developer causes no such error, maybe this post will help.
By default, SQLcl and SQL*Plus don’t allow blank lines in SQL statements. However, you can change this with the SET SQLBLANKLINES command.
In SQL Server, you can use the sp_rename stored procedure to rename an object, including a column.
In SQL, you can join three tables or more by adding another join after the first one.
You can also run nested joins by specifying one join as the join condition for another.
Continue readingIn SQL, you create a relationship by creating a foreign key constraint.
More specifically, you have a parent table and a child table. The parent contains the primary key, and the child table contains a foreign key that references the primary key of the parent table.
When you use SQL to create a relationship, you can create the relationship at the time you create the table, or you can create it later (by altering the table). This article covers both scenarios.
Continue readingThis page contains direct links to SQL tutorials and reference articles for beginners.
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