To join 2 columns and extract year and count in PostreSQL, you can use the following SQL query:
SELECT EXTRACT(year FROM date_column) AS year, COUNT(*) AS count FROM your_table GROUP BY year;
This query will extract the year from a date column in your table, and then count the occurrences of each unique year. The result will be a list of years and their corresponding count in the table.
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What is a common table expression (CTE) in PostgreSQL?
A common table expression (CTE) in PostgreSQL is a temporary result set that can be referenced within a SELECT, INSERT, UPDATE, or DELETE statement. It allows you to define a query that can be used within another query, making complex queries easier to read and maintain. CTEs are defined using the WITH keyword followed by the name of the CTE and the query that defines it.
What is an index in PostgreSQL?
In PostgreSQL, an index is a database object used to speed up the retrieval of rows from a table. It is a data structure that allows for quick lookup of data based on specific columns in a table. An index can be created on one or more columns of a table, enabling faster searching and sorting operations. By using indexes, queries can be processed more efficiently, resulting in faster response times for database operations.
What is a unique constraint in PostgreSQL?
In PostgreSQL, a unique constraint is a rule that ensures that all values in a column or a group of columns are unique, meaning that there are no duplicate values. This constraint is used to enforce data integrity and prevent duplicate entries in a table. When a unique constraint is defined on one or more columns, PostgreSQL automatically creates a unique index on those columns to enforce the uniqueness requirement.
What is a self-join in PostgreSQL?
A self-join in PostgreSQL is a type of join where a table is joined to itself. This is typically done when you want to retrieve related data from the same table.
For example, if you have a table of employees with a column for the manager's employee ID, you could use a self-join to retrieve the name of each employee along with the name of their manager. This would involve joining the employee table to itself on the manager's employee ID.
Self-joins are useful for hierarchical data structures or when you have a table that contains both parent and child records.
