Unknown column in a table Create schema Nevy;

create table sailor
(
  sid INT(5) NOT NULL,
  sname Varchar(20) NOT NULL,
  rating INT(5) NOT NULL,
  age INT(5) Not NULL,
  primary key (sid));

Create table reserve
(
  sid INT(5) NOT NULL,
  bid INT(5) NOT NULL,
  DAYT date NOT NULL,
  primary key (sid,bid,dayt),
  foreign key (bid) references boat(bid),
  foreign key (sid) references sailor(sid)
);

Create Table boat
(
  bid INT(5) NOT NULL,
  bname VARCHAR(10) NOT NULL,
  color VARCHAR(10) NOT NULL,
  primary key (bid) );


INSERT INTO nevy.sailor 
values(22,'Dustin',7,45),(29,'Brutus',1,33),
  (31,'Lubber',8,55.5),(32,'Andy',8,25.5),
  (58,'Rusty',10,35),(64,'Horatio',7,35),
  (71,'zorba',10,16), (74,'Horatio',9,35),
  (85,'Art',3,25.5),(95,'Bob',3,63.5);

INSERT INTO nevy.boat 
values(101,'interlake','blue'),(102,'Interlake','red'),
  (103,'Clipper','green'),(104,'Marine','Red');

INSERT INTO nevy.reserve 
values (22,101,'1998-10-10'),(22,102,'1998-10-10'),
  (22,103,'1998-8-10'),(22,104,'1998-7-10'),
  (31,102,'1998-10-11'),(31,103,'1998-6-11'),
  (31,104,'1998-12-11'),(64,101,'1998-5-9'),
  (64,102,'1998-8-9'),(74,103,'1998-8-9');

select sname 
from sailor 
Where sailor.sid= reserve.sid 
  AND bid=103;


When I execute the last select statement , it gives me the error 


  unknown column: reserve.sid


I don't know why this error is being generated.
The error is being generated because you currently aren't joining your sailor table to reserve so the engine doesn't have any reference to the reserve table.

In order to make the reserve table available, you'll need to JOIN the tables on the sid column similar to:

select s.sname 
from sailor s
inner join reserve r
  on s.sid= r.sid 
where r.bid=103;


Or if you just want to return the sname that exists in the reserve table, then you could use EXISTS:

select s.sname
from sailor s
where exists (select 1 
              from reserve r
              where bid = 103
                and s.sid = r.sid);


If you needed to include all 3 tables in a single query, then you would use:

select s.name
from sailor s
inner join reserve r
  on s.sid = r.sid
inner join boat b
  on r.bid = b.bid