How to avoid top 5 mistakes in SQL

Let’s discuss some tips and tricks to avoid major mistakes in SQL

Never use Select *

• select *, gets the data from all the columns, which increases the latency and is expensive for huge data.
• Instead, get only the required fields, which limits the size of each record

Use EXISTS() Instead of COUNT()

• If you want to know whether a record/result exists or not, it is better to use exists(), rather than count()
• count(), will browse the entire table to get you the number of records.
• Whereas, exists(), will get back to you, when it finds the first record for the query, saving you time and computing.

Functions on indexed columns is useless

• Using functions on indexed columns, while querying, will make the indexes remain useless.
• In order to use indexes, we need to avoid adding functions on the indexed columns.

LIMIT Statements

• Most common used pagination is LIMIT, OFFSET, which is not optimal.
• It is fast for smaller and immediate sets like “LIMIT 0, 10”
• But when OFFSET is changed to 1000000, it takes too long, since database doesn’t know where 1000000th record exists, it starts from scratch till it finds 1000000th row.
• Better approach would be to add another filter, mostly indexed column, like below

Use GROUP BY Instead of DISTINCT

• The distinct is an expensive operation, and doesn’t use indexes if available.
• Faster and easier way to do the same is to use group by.

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In Conclusion…

I haven’t covered many major SQL pitfalls. But these are some of the top mistakes, even made by experienced developers. Now you get to avoid them 😁

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