MYSQL from syntax with comma, MYSQL Where with #s, find_in_set, length, concat

 MYSQL from syntax with comma 

from syntax with comma means inner join with no ON statement. Meaning it will return every matching pair for both tables. If one table A contains only 1 column and 1 value, meaning every row from other table B will be paired with this row, and returned, the value for  table A for paried rows will be null.

If table A conatins no row, then nothing will be returned from this inner join.

Detailed explanation from stack over flow :

https://stackoverflow.com/questions/26465412/how-does-mysql-from-clause-work-with-multiple-table-names

Your query with the empty result set is this, elaborated for readability

SELECT ord.* 
  FROM orders AS ord, order_hotels  AS oh

It is the old-timey comma-separated list of tables, syntax for this query

     SELECT ord.* 
       FROM orders AS ord
 INNER JOIN order_hotels  AS oh   

This query, an INNER JOIN without any ON clause, returns one row for each possible pairing of rows in orders and order_hotels. You have stated that order_hotels contains no rows. So, this query is dutifully returning every possible pairing, which is no rows.

MYSQL where with #s

where 1(means 1=1) or where 1 AND 4, or where 4 AND 5, means where has no effect, all rows will be returned.

where 0 means(0=0) or where 0 and 1. or where 1 and 0, means no rows will be returned.

stack over flow explanation 

https://stackoverflow.com/questions/1983655/importance-of-where-1-in-mysql-queries

MYSQL find_in_set function

The FIND_IN_SET() function returns the position of a string within a list of strings.

Poistion starts at 1 instead of 0

https://www.w3schools.com/sql/trymysql.asp?filename=trysql_func_mysql_find_in_set

https://www.w3schools.com/sql/trymysql.asp?filename=trysql_func_mysql_find_in_set

If not exist, will return 0

https://www.w3schools.com/sql/trymysql.asp?filename=trysql_func_mysql_find_in_set2

will return empty if string is null

https://www.w3schools.com/sql/trymysql.asp?filename=trysql_func_mysql_find_in_set3

MYSQL length function

The LENGTH() function returns the length of a string (in bytes).

https://www.w3schools.com/sql/trymysql.asp?filename=trysql_func_mysql_length

SELECT LENGTH("SQL Tutorial") AS LengthOfString;

returns 12, 1 byte for each char(11), 1 byte for space

https://www.w3schools.com/sql/trymysql.asp?filename=trysql_func_mysql_length

MYSQL concat function 

Add string together

https://www.w3schools.com/sql/func_mysql_concat.asp

SELECT CONCAT("SQL ", "Tutorial ", "is ", "fun!") AS ConcatenatedString;

SQL Tutorial is fun!

https://www.w3schools.com/sql/trymysql.asp?filename=trysql_func_mysql_concat