Thread by @rfeers

Ever felt confused by SQL's execution flow? 🤔

Then you better stay with me!

Today let's exemplify SQL's execution order with a simple query👇🏻

1️⃣ 𝗦𝗧𝗔𝗥𝗧𝗜𝗡𝗚 𝗙𝗥𝗢𝗠 𝗢𝗨𝗥 𝗥𝗔𝗪 𝗧𝗔𝗕𝗟𝗘
We use a dummy table with the salary of employees depending on their field and experience,

🎯 Our main goal?
Understand the field that earns the most.

2️⃣ 𝗦𝗤𝗟 𝗤𝗨𝗘𝗥𝗬 𝗦𝗧𝗥𝗨𝗖𝗧𝗨𝗥𝗘 (to use)
We define a query to obtain our goal data.

3️⃣ 𝗦𝗤𝗟 𝗔𝗦 𝗔 𝗗𝗘𝗖𝗟𝗔𝗥𝗔𝗧𝗜𝗩𝗘 𝗟𝗔𝗡𝗚𝗨𝗔𝗚𝗘
SQL expects statements to be written in a specific order ...

but their evaluation sequence differs.

𝗦𝗧𝗘𝗣 1 - 𝗙𝗥𝗢𝗠
First SQL determines the table where to take the data from.

In our case our dummy table salaries with all the info it contains.

𝗦𝗧𝗘𝗣 2 - 𝗪𝗛𝗘𝗥𝗘
Here data is filtered.

This is where we specify conditions to narrow down our results.

Think of it as our detective's magnifying glass! 🔍

𝗦𝗧𝗘𝗣 3 - 𝗚𝗥𝗢𝗨𝗣 𝗕𝗬
Grouping time!

This step clusters similar data together based on a specified column.

Perfect for summarizing data like employee fields! 📊

𝗦𝗧𝗘𝗣 4 - 𝗛𝗔𝗩𝗜𝗡𝗚
It's like WHERE's big sibling.

HAVING filters groups, especially useful after GROUP BY.

Only the groups meeting our criteria will pass this gate.

𝗦𝗧𝗘𝗣 5 - 𝗦𝗘𝗟𝗘𝗖𝗧
The star of the show!

Here we pick the columns we want to see in our final output.

Like an artist choosing colors for their painting 🎨

𝗦𝗧𝗘𝗣 6 - 𝗢𝗥𝗗𝗘𝗥 𝗕𝗬
Now we arrange our results.

This is where we decide the sorting order, like arranging books on a shelf. 📚

𝗦𝗧𝗘𝗣 7 - 𝗟𝗜𝗠𝗜𝗧
The final touch!

Here we can limit the number of rows in our result set.

It's like picking the top 10 hits of the year. 🌟

4️⃣ 𝗙𝗜𝗡𝗔𝗟 𝗥𝗘𝗦𝗨𝗟𝗧
And voilà....!

we have our neatly arranged and insightful data, ready for action!

Next week we will see how a JOIN command would affect this flow!

Still no clear how SQL's execution flow works?

Check my previous thread to further understand it! 👇🏻

And that's all for now

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