Similarities and differences between WHERE and HAVING in SQL grouping

When we use SQL for grouping, we can use GROUP BY to achieve:

select product_id,sum(amt)
from a_fin_factline
group by 1

The program intent is clear: aggregate sales by product.

When we need to limit the grouping, such as excluding groups with null products, we can do it in two ways:

select product_id,sum(amt)
from a_fin_factline
group by 1
having product_id is not null;

select product_id,sum(amt)
from a_fin_factline
where product_id is not null
group by 1;

Execution time comparison in mysql:

select product_id,sum(amt)
from a_fin_factline
group by 1
having product_id is not null;
Affected rows: 0
Time: 1.086s

select product_id,sum(amt)
from a_fin_factline
where product_id is not null
group by 1;
Affected rows: 0
Time: 0.098s

It can be seen that it is more efficient to use WHERE.

The differences between the two methods are summarized as follows:

• Syntactically: WHERE comes before GROUP BY, HAVING comes after
• Execution plan: WHERE makes the limit happen before grouping, HAVING is after grouping
• Efficiency: using WHERE makes the target result set smaller, and the efficiency of grouping and aggregation is high; when using HAVING, the result set remains unchanged, and WHERE is more efficient
• Other: WHERE can use any field in the table to limit, and HAVING must be the field after SELECT