SELECT * FROM tableName WHERE 'num+sometext' - mysql

Question

I am curious to know , how these queries work in database engine. Specially my focus is on line no 4 and 6.
1.SELECT * FROM tableName
2.SELECT * FROM tableName WHERE 1
3.SELECT * FROM tableName WHERE 123
4.SELECT * FROM tableName WHERE '2xyz'
5.SELECT * FROM tableName WHERE ''
6.SELECT * FROM tableName WHERE 'xyz'
In the above queries 1,2,3,4 are producing same result but 5 and 6 are not producing any result. why? what is diff b/w WHERE '2xyx' and 'xyz'?
How '2xyz' implicitly converted into 2 ?

Answer 1

The where clause evaluates to boolean true (not 0) or false (0) in order to decide if a record is in or out of the resultset.
Cases 2 and 3 retrive all records because the non zero numbers evaluate to true.
Case 3 retrieves all records because during the implicit string to number conversion mysql evaluates the string from left to right character by character. As long as as the characters can be evaluated as a number, mysql will take their value. This includes chopping of leading spaces, interpreting plus or minus signs, decimal points, and so on. So, the string '2xyx' is interpreted as 2, thus boolean true.
Case 4 and 5 do not retrieve any records because the strings' leftmost character cannot be evaluated as a number, so the conversion returns 0, thus boolean false.
Unfortunately, the implicit string to number conversion is not really documented in the MySQL manual. Most of the rules, however, can be deducted from the following part in Type Conversion in Expression Evaluation section of the manual:
For comparisons of a string column with a number, MySQL cannot use an
index on the column to look up the value quickly. If str_col is an
indexed string column, the index cannot be used when performing the
lookup in the following statement:
SELECT * FROM tbl_name WHERE str_col=1;1
The reason for this is that there are many different strings that may
convert to the value 1, such as '1', ' 1', or '1a'.

Answer 2

It is WHERE <condition>, e.g. WHERE col1 = 123. What you have instead is WHERE <number> or WHERE <string>, so there is an expression missing.
What would be possible though is WHERE <boolean>, and this is what MySQL expects. And as MySQL treats booleans like numbers (0 = false, other numbers = true), it looks for a number.
1 and 123 are numbers that result in true.
'2xyz' gets converted to 2, i.e. true.
'' and 'xyz' get converted to 0, i.e. false.
So the latter give you an empty result set as the where condition renders false.

Answer 3

In statement SELECT ... WHERE [where_condition], in where_condition expression you can use any of the functions and operators that MySQL supports, in which string literal is also supported. For strings the comparisons are based on the numeric values of the string unit. When evaluating an expression type conversion is also done.
So in above case the 2xyz gets converted to 2 which is true and hence you get all the records. Its like
SELECT * FROM tableName WHERE 2
and the string xyz gets converted to 0 which is false due to which you get empty result set. Its same as:
SELECT * FROM tableName WHERE 0

Answer 4

Where having condition value. Mysql any number treat as TRUE value and null or '' treat as FALSE.
1.SELECT * FROM tableName -- all record return
2.SELECT * FROM tableName WHERE 1 -- Return true value
3.SELECT * FROM tableName WHERE 123 -- Return true value
4.SELECT * FROM tableName WHERE '2xyz' -- Return true value because there first character is number
5.SELECT * FROM tableName WHERE '' -- false value return
6.SELECT * FROM tableName WHERE 'xyz' -- false value return