1. MySQL Explain examples

2. create table customers ( custid integer primary key, renew date, magid int );
create table magazines ( magid int primary key, cost int );
select sum(cost) from magazines where
magid in (select magid from customers);
select sum(cost) from customers join magazines on customers.magid = magazines.magid;

3. create table customers ( custid integer primary key, renew date, magid int );
create table magazines ( magid int primary key, cost int );
alter table customers add constraint foreign key(magid) references magazines(magid);
select sum(cost) from magazines where magid in ( select magid from customers );
select sum(cost) from customers join magazines on customers.magid = magazines.magid;

4. Using index; FirstMatch(magazines)
create table customers ( custid integer primary key, renew date, magid int );
create table magazines ( magid int primary key, cost int );
alter table customers add constraint foreign key(magid) references magazines(magid);
select sum(cost) from magazines where magid in ( select magid from customers ); id
select_type
table
type
possible_keys
key
key_len
ref
rows
Extra 1
SIMPLE
magazines
index
PRIMARY,FK2
FK2 9
NULL
Using index
customers
FK1 5
magazines.magid
Using index; FirstMatch(magazines)