How to write conditional judgment in sql

Conditional judgments in SQL can be used to filter data and only return rows that meet specified conditions. The WHERE clause is used to filter rows, and the HAVING clause is used to filter rows in the group result set generated by the aggregate function. Conditional judgment uses operators such as equal to, not equal to, greater than, less than, and logical operators such as AND, OR, and NOT. Conditions can be nested to create more complex filters, and the precedence of nested conditions follows the parenthesized condition, NOT operator, AND operator, OR operator.

Conditional judgment in SQL

Conditional judgment is used in SQL queries to filter data and only return data that satisfies specific conditional row. There are two main conditional judgments in SQL:

• WHERE clause: is used to filter rows and only return rows that meet the specified conditions.
• HAVING clause: Used to filter rows in a group result set produced by an aggregate function such as SUM, COUNT, or AVG.

WHERE clause

The WHERE clause is located at the end of the SELECT statement and is used to specify filter conditions. The condition consists of a logical expression that evaluates the column of the row and returns TRUE or FALSE.

Grammar:

<code class="sql">SELECT column_name(s)
FROM table_name
WHERE condition;</code>

Example:

<code class="sql">SELECT *
FROM customers
WHERE age > 18;</code>

HAVING clause

Syntax:

<code class="sql">SELECT column_name(s)
FROM table_name
GROUP BY group_column(s)
HAVING condition;</code>

Example:

<code class="sql">SELECT department_id, AVG(salary)
FROM employees
GROUP BY department_id
HAVING AVG(salary) > 50000;</code>

Conditional operator

• is equal to: =
• is not equal to: <> or !=
• Greater than: >
• Less than: <
• Greater than or equal to: > =
• Less than or equal to: <=
• LIKE: is used to match patterns (for example, "LIKE '%john%'" matches Rows containing the string "john")

Logical Operators

• AND: Logical AND operator, returns TRUE only when both conditions are TRUE.
• OR: Logical OR operator, returns TRUE as long as one condition is TRUE.
• NOT: Logical NOT operator, inverts the conditional result.

Nested conditions

Conditions within brackets
1. NOT operator
2. AND operator
3. OR operator

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