Start from Scratch: A Step-by-Step Guide for Learning SQL Queries

Complex queries might be scary, but this guide will show you how to work more readily with SQL queries.

SQL Queries, which stands for Structured Query Language, is a programming language that is used to manage and manipulate data in a relational database management system (RDBMS). It is a widely used language in many organizations for ensuring smooth data access. SQL is a required skill in many job postings due to its widespread use, which makes it important for people to learn.

One of the common challenges for those learning SQL is understanding queries, especially when they are written by someone else. In companies, we often work as a team and need to read and comprehend SQL queries. Therefore, it is crucial to practice deconstructing and understanding SQL queries.

This article will guide you through a step-by-step process of reading and understanding SQL queries. How can we achieve this? Let’s find out.

1. Understand The Structure Of SQL Queries

When encountering a SQL query, the first thing we need to do is comprehend its overall aim. The broad aim does not imply a complete understanding of the query’s structure; rather, it refers to the overall flows.

To comprehend the general SQL query, we need to first understand the standard SQL queries.  Most SQL queries begin with the SELECT clause and continue with the FROM clause. From there, the queries are typically followed by JOIN, WHERE, GROUP BY, ORDER BY, and HAVING clauses.

The clauses mentioned above are common in the SQL queries we need to comprehend. Each clause has the following functions:

• SELECT: Columns to extract from the table
• FROM: Table from which the data was obtained
• JOIN: Combine tables using the supplied identifier.
• WHERE: Data filters based on the condition.
• GROUP BY: Organize the data by column value and enable aggregation.
• ORDER BY: Arrange the data result order by the specific column.
• HAVING: The filter condition for the aggregation function, which cannot be provided WHERE

These are the usual clauses that you should expect to see while first learning about the generic SQL query format. Let’s utilize the example code to understand more.

SELECT 
  customers.name, 
  purchases.product, 
  SUM(price) as total_price 
FROM 
  purchases 
  JOIN customers ON purchases.cust_id = customers.id 
WHERE 
  purchases.category = 'kitchen' 
GROUP BY 
  customers.name, 
  purchases.product 
HAVING 
  total_price > 10000 
ORDER BY 
  total_price DESC;

When reviewing the query above, try to locate the standard clauses. The clause would explain what data was selected (SELECT), where it came from (FROM and JOIN), and the conditions (WHERE, GROUP BY, ORDER BY, and HAVING).

For example, reading the query above would provide you a comprehension of the following:

1. We attempt to obtain three types of data: the name from a table named customers, the product from a table called purchases, and the aggregation of price columns that do not identify the table’s origin and are referred to as total price (information from clause SELECT).
2. The total data would come from the purchases and customers tables, which would be joined using the cust_id column from purchases and the id column from the customer’s table (Information from clause FROM and JOIN).
3. To use the aggregation function, select data with the category column value ‘kitchen’ from the purchases table (Information from clause WHERE).
4. Group by the name and product column from the respective table (Information from clause GROUP BY).
5. Filter the result sum for total price greater than 10000 (Information from clause HAVING).
6. Order the data descending by total price (information from clause ORDER BY).

That is the general SQL query format you must understand and identify. From there, we may investigate further using the advanced query. Let’s go on to the next step.

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2. The Final Select

There will be times when you come across a complex query with multiple SELECT clauses. In this scenario, we need to understand the query’s ultimate result, or the final (first) SELECT statement. The trick is to understand what the query output is intended to be.

Let’s Try out one more code:

WITH customerspending AS (
  SELECT 
    customers.id, 
    SUM(purchases.price) as total_spending 
  FROM 
    purchases 
    JOIN customers ON purchases.cust_id = customers.id 
  GROUP BY 
    customers.id
) 
SELECT 
  c.name, 
  pd.product, 
  pd.total_product_price, 
  cs.total_spending 
FROM 
  (
    SELECT 
      purchases.cust_id, 
      purchases.product, 
      SUM(purchases.price) as total_product_price 
    FROM 
      purchases 
    WHERE 
      purchases.category = 'kitchen' 
    GROUP BY 
      purchases.cust_id, 
      purchases.product 
    HAVING 
      SUM(purchases.price) > 10000
  ) AS pd 
  JOIN customers c ON pd.cust_id = c.id 
  JOIN customerspending cs ON c.id = cs.id 
ORDER BY 
  pd.total_product_price DESC;

The query is more complex and lengthy now, but the primary attention should be on the final SELECT, which appears to attempt to produce the customer’s entire spending and purchase history. Try to determine what the desired final result is and break it down from there.

3. The Final Condition Clause

The inquiries provide us with information into what the result should be. The next step is to determine the conditions for the final SELECT statement. The conditions clause, which included WHERE, GROUP BY, ORDER BY, and HAVING, determined the overall data result.

Try to read and grasp the conditions of our query, and we will have a better understanding of the final answer. For example, in our previous SQL query, the final condition is simply ORDER BY. This signifies that the final result will be sorted by total product price in descending order.

Knowing the final conditions will help you grasp a substantial portion of the query and the overall query intent.

4. The Final Join

First and foremost, we must understand the source of the data. After we’ve determined which data to select and under what conditions, we need to understand the source. The last JOIN clause would help us understand how the tables connect and how data flows.

For example, the prior complex query indicates that we used Join twice. This signifies that we used at least three data sources to produce the final result. This information will be required in the subsequent steps to better understand how each data source is obtained, particularly when the data source is derived from the subquery.

5. Reverse Order Reading And Repeat

After we’ve determined what the end result should be and where it comes from, we need to go deeper into the intricacies. From here, we’d go back and look at each subquery to figure out why it’s set up that way.

However, we do not attempt to examine them in top-down frameworks. Instead, we should start with the subqueries that are closest to the final result and work our way up to the one that is the furthest away. We should first try to understand the code in the example

SELECT 
  purchases.cust_id, 
  purchases.product, 
  SUM(purchases.price) as total_product_price 
FROM 
  purchases 
WHERE 
  purchases.category = 'kitchen' 
GROUP BY 
  purchases.cust_id, 
  purchases.product 
HAVING 
  SUM(purchases.price) > 10000

Then, we are moving to the furthest code which is this one:

WITH customerspending AS (
  SELECT 
    customers.id, 
    SUM(purchases.price) as total_spending 
  FROM 
    purchases 
    JOIN customers ON purchases.cust_id = customers.id 
  GROUP BY 
    customers.id
)

We can plainly see the author’s thought process when we split out each subquery from closest to the result to farthest.

If you are having trouble comprehending each subquery, try repeating the method above. With a little practice, you’ll get better at reading and understanding the query.

Conclusion

Being able to read and interpret SQL queries is an important skill that is required by every firm in the present era. If you wish to understand complex SQL queries more easily, it is recommended that you follow the step-by-step guide given below. The steps include understanding the general SQL query structure, comprehending the Final Select, understanding the Final Condition Clause, and reading the last join in reverse order and repeating it until you understand it completely.

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