left Join ON condition and Select where condition difference - mysql

Question

I have three tables {animal, food, animal_food}
DROP TABLE IF EXISTS `tbl_animal`;
CREATE TABLE `tbl_animal` (
id_animal INTEGER NOT NULL PRIMARY KEY AUTO_INCREMENT,
name VARCHAR(25) NOT NULL DEFAULT "no name",
sex CHAR(1) NOT NULL DEFAULT "M",
size VARCHAR(10) NOT NULL DEFAULT "Mini",
age VARCHAR(10) NOT NULL DEFAULT "born",
hair VARCHAR(5 ) NOT NULL DEFAULT "short",
color VARCHAR(25) NOT NULL DEFAULT "not defined",
FOREIGN KEY (sex) REFERENCES `tbl_sexes` (sex),
FOREIGN KEY (tamanio) REFERENCES `tbl_sizes` (size),
FOREIGN KEY (age) REFERENCES `tbl_ages` (age),
FOREIGN KEY (hair) REFERENCES `tbl_hair_length` (hair_length),
CONSTRAINT `uc_Info_Animal` UNIQUE (`id_animal`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
DROP TABLE IF EXISTS `tbl_food`;
CREATE TABLE `tbl_food` (
id_food INTEGER NOT NULL PRIMARY KEY,
type_food VARCHAR(20) NOT NULL DEFAULT "Other",
label VARCHAR(50) NOT NULL,
CONSTRAINT `uc_Info_Food` UNIQUE (`id_food`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
DROP TABLE IF EXISTS `animal_food`;
CREATE TABLE `animal_food` (
id_animal INTEGER NOT NULL,
food VARCHAR(50) NOT NULL DEFAULT "",
quantity VARCHAR(50) NOT NULL DEFAULT "",
times VARCHAR(50) NOT NULL DEFAULT "",
description VARCHAR(50) NOT NULL DEFAULT "",
date_last DATE DEFAULT '0000-00-00 00:00:00',
date_water DATE DEFAULT '0000-00-00 00:00:00',
CONSTRAINT fk_ID_Animal_Food FOREIGN KEY (id_animal) REFERENCES `tbl_animal`(id_animal)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
And I have a view where I select the values columns in animal and animal_food depending on ID
CREATE VIEW `CAT_animal_food` AS
SELECT a.name, a.sex,a.size,a.age,a.hair,a.color,
a_f.*
FROM `tbl_animal` a, `animal_food` a_f
WHERE a.id_animal = a_f.id_animal;
What would be better to create a view like above or, to join these animal and animal_food tables?
SELECT ...
FROM A.table t1
JOIN B.table2 t2 ON t2.column = t1.col
What is really the diference between that kind of view and a left join for example?

Answer 1

The only difference between the two SELECT statements is the syntax style. Both perform INNER JOINS. In other words, this style uses what is called "implicit" syntax:
SELECT ...
FROM A.table t1, B.table2 t2
WHERE t2.column = t1.col
It is "implicit" because the join condition is implied by the WHERE clause. This version uses "explicit" syntax:
SELECT ...
FROM A.table t1
JOIN B.table2 t2 ON t2.column = t1.col
Most people prefer to see "explicit" syntax because it make your code easier to follow; the join condition is explicitly understood and any WHERE clause is obvious.
None of this is related to LEFT JOINS of course. Here is a famous link with a great visual description of join types.

Answer 2

There is a big difference.
The old school style using a list of tables only allows inner joins.
Further, placing non-key conditions in the ON clause of a proper join allows greater performance and capability that a where clause can not deliver. The primary reason for this is that the ON clause is evaluated as the join is being made, but the WHERE clause is evaluated after all joins have been made.
The subject is too complex to do justice here.