Database.Guide – Page 187

MySQL UNION Clause

February 16, 2022 by Ian

In MySQL, the UNION clause combines the results from multiple queries into a single result set.

3 Ways to Select the Row with the Minimum Value in SQL

February 15, 2022 by Ian

Here are three examples of using SQL to find and select the row with the minimum value in a given column.

The examples work in most major RDBMSs, including MySQL, MariaDB, PostgreSQL, SQLite, Oracle, and SQL Server.

SHOW TABLES in MariaDB

November 15, 2025February 15, 2022 by Ian

In MariaDB, SHOW TABLES is an administrative statement that lists the non-TEMPORARY tables, sequences and views in a given database.

Oracle INTERSECT Operator Explained

February 15, 2022 by Ian

In Oracle Database, the INTERSECT operator is used to create a compound query that returns the intersection of the results of the left and right SELECT statements. In other words, it combines two queries, but returns only those rows that are returned in both queries.

6 Ways to Select Duplicate Rows in Oracle

July 23, 2023February 14, 2022 by Ian

The following examples return duplicate rows from an Oracle Database table.

Fix “ERROR: missing FROM-clause entry for table” in PostgreSQL when using UNION, EXCEPT, or INTERSECT

February 14, 2022 by Ian

If you’re getting “ERROR: missing FROM-clause entry for table” in PostgreSQL when using an operator such as UNION, INTERSECT, or EXCEPT, it could be because you’re qualifying a column name with its table name.

To fix this, either remove the table name or use a column alias.

Oracle MINUS Operator Explained

February 14, 2022 by Ian

In Oracle Database, the MINUS operator is used to return only unique rows returned by the first query but not by the second.

4 Ways to Find Rows that Contain Uppercase Letters in Oracle

February 13, 2022 by Ian

Here are four options for returning rows that contain uppercase characters in Oracle Database.

Fix “ERROR: each UNION query must have the same number of columns” in PostgreSQL

February 13, 2022 by Ian

When using the UNION operator in PostgreSQL, if you encounter an error that reads “ERROR: each UNION query must have the same number of columns“, it’s because there’s a mismatch in the number of columns returned by the queries on either side of the UNION operator.

This error occurs when the number of columns returned by each SELECT statement is different.

The way to fix this is to ensure that both SELECT statements return the same number of columns.

How UNION Works in PostgreSQL

February 13, 2022 by Ian

In PostgreSQL, the UNION operator combines the results from multiple queries into a single result set.