How to implement selection operation in sql

In SQL, the selection operation is used to extract specific rows from a table based on specified conditions. The main methods include: WHERE clause: Specify a condition to select rows that meet the condition. HAVING clause: Filter the grouped data, and the condition refers to the aggregate function.

Selection operation in SQL

Selection operation, also known as filter operation, is used in SQL Extract rows from a table that meet certain criteria. There are two main ways to implement selection operations:

1. WHERE clause

The WHERE clause is the most common method for selection operations. It allows you to specify a condition and select only rows that meet that condition.

Syntax:

<code>SELECT * FROM table_name WHERE condition;</code>

For example:

<code>SELECT * FROM customers WHERE age > 30;</code>

This query will select all rows where the age column is greater than 30.

2. HAVING clause

The HAVING clause is similar to the WHERE clause, but it is used to filter grouped data. The conditions in the HAVING clause must reference an aggregate function (such as SUM, COUNT, AVG) that summarizes the grouped data.

Syntax:

<code>SELECT aggregate_function(column_name) AS alias
FROM table_name
GROUP BY group_by_column
HAVING condition;</code>

For example:

<code>SELECT SUM(sales) AS total_sales
FROM orders
GROUP BY customer_id
HAVING SUM(sales) > 1000;</code>

This query will find every customer with a total sales volume greater than 1000.

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