[MySQL] An example of subquery

Assume that the table my_tbl contains three fields A, B, and C. Now we need to query the minimum value of Column B under each different value of column A in the table.
For example, the table record is: A B C 1 3 'cd' 2 3 'nhd' 1 5 'bg' 2 6 'cds' 1 7 'kiy' 3 7 'Password' 3 8 'ndf'
Expected result: A B C 1 3 'cd' 2 3 'nhd' 3 7 'Password'
(1) One method is to first find the minimum value of B under each a value, and then query all records that meet the requirements based on these minimum values. The statement for querying SQL statements that meet the minimum B value is as follows: Select a. * From my_tbl as a where a. B = (select Min (B) from my_tbl as B where B. A = A. );
Because of the nested query and intersection, the intermediate results were not calculated for an hour in the case of 0.8 million records (I doubt where I wrote the error ); the record quantity will not be discussed later.
(2) The above method is a disaster and can only be discarded. The specific logic is: first group by column A and column B, and then select the record with the smallest column B value in each group to generate the result set. The SQL statement is written as follows: SelectA, B, C, count ()From (select a, B, c from my_tbl group by a, B) as a group
By;
After the query is executed, it takes only 1.1 seconds.

Once again, it is proved that different SQL query policies can directly lead to huge performance differences.