How Can I Optimize SQL Queries Using Joins, Unions, Subqueries, and Advanced Functions?

Part 1 - Joins and Unions

Joining Two or More Tables Using an Inner Join

select
    a.ID,
    b.model
from
    cars a
        join models b
            on a.model=b.ID

Using a Union Query

select
    a.ID,
    b.model,
    c.color
from
    cars a
        join models b
            on a.model=b.ID
        join colors c
            on a.color=c.ID
where
    b.ID=1
union all
select
    a.ID,
    b.model,
    c.color
from
    cars a
        join models b
            on a.model=b.ID
        join colors c
            on a.color=c.ID
where
    b.ID=3

Left and Right Outer Joins

select
    a.brand
from
    brands a
        left outer join cars b
            on a.ID=b.brand

Intersect Queries

select
    *
from
    colors
where
    ID>2
intersect
select
    *
from
    colors
where
    id<h2 id="Part-Subqueries">Part 2 - Subqueries</h2><p><strong>What they are, where they can be used and what to watch out for</strong></p><p>A subquery is a select statement that is nested within another select statement. Subqueries can be used to perform complex data retrieval operations, such as filtering, sorting, and aggregation.</p><p><strong>Where they can be used</strong></p><p>Subqueries can be used in the following places:</p>

• In the WHERE clause to filter the rows that are returned by the outer query.
• In the HAVING clause to filter the groups of rows that are returned by the outer query.
• In the SELECT clause to specify the columns that are returned by the outer query.
• In the FROM clause to specify the tables that are joined by the outer query.

What to watch out for

When using subqueries, it is important to be aware of the following:

• Subqueries can be expensive to execute, so it is important to use them only when necessary.
• Subqueries can be difficult to read and understand, so it is important to document them well.
• Subqueries can be vulnerable to SQL injection attacks, so it is important to use parameterized queries when using subqueries in dynamic SQL.

Part 3 - Tricks and Efficient Code

Tricks

• Use aliases for table names to make your queries easier to read and understand.
• Use parentheses to group your subqueries to make them easier to read and understand.
• Use the EXPLAIN statement to see how your queries are being executed by the database.
• Use indexes to improve the performance of your queries.

Efficient Code

• Use the correct data types for your columns.
• Avoid using SELECT * in your queries.
• Use the WHERE clause to filter the rows that are returned by your queries.
• Use the ORDER BY clause to sort the rows that are returned by your queries.
• Use the LIMIT clause to limit the number of rows that are returned by your queries.

Part 4 - Subqueries in the From Clause

Subqueries can be used in the FROM clause to specify the tables that are joined by the outer query. This is known as a derived table. Derived tables can be used to perform complex data retrieval operations, such as filtering, sorting, and aggregation.

The following example shows how to use a subquery in the FROM clause to filter the rows that are returned by the outer query:

select
    a.ID,
    b.model
from
    cars a
        join models b
            on a.model=b.ID

Part 5 - Mixed Bag of John's Tricks

John's Tricks

• Use the CASE statement to conditionally evaluate expressions.
• Use the COALESCE function to return the first non-NULL value in a list of expressions.
• Use the GREATEST function to return the largest value in a list of expressions.
• Use the LEAST function to return the smallest value in a list of expressions.
• Use the MOD function to calculate the remainder of a division operation.
• Use the NOW function to get the current date and time.
• Use the RAND function to generate a random number.
• Use the ROUND function to round a number to the nearest integer.
• Use the TRUNCATE function to truncate a number to a specified number of decimal places.

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