SQL如何处理int/string操作数与列之间的转换

Question

I am trying to understand how comparison is done when the operand is integer and column type is string and vice versa.

Sample table

My table is as follows:

mysql> DESCRIBE tbl1;
+-------+--------------+------+-----+---------+-------+
| Field | Type         | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| col1  | varchar(255) | YES  |     | NULL    |       |
| col2  | int(11)      | YES  |     | NULL    |       |
| col3  | int(11)      | YES  |     | NULL    |       |
| col4  | varchar(255) | YES  |     | NULL    |       |
+-------+--------------+------+-----+---------+-------+

Its content is as follows:

mysql> SELECT * FROM tbl1;
+------+------+------+------+
| col1 | col2 | col3 | col4 |
+------+------+------+------+
| val1 | NULL |    7 | 7    |
| val4 | NULL | NULL | abc  |
| val2 |    2 |    8 | 8    |
| val3 |    3 |   18 | 18   |
+------+------+------+------+

Some queries

1. Comparing string with INT column:

    mysql> SELECT * FROM tbl1 WHERE "9" > col3; +------+------+------+------+ | col1 | col2 | col3 | col4 | +------+------+------+------+ | val1 | NULL | 7 | 7 | | val2 | 2 | 8 | 8 | +------+------+------+------+

2. Comparing integer with VARCHAR column:

    mysql> SELECT * FROM tbl1 WHERE 9 > col4; +------+------+------+------+ | col1 | col2 | col3 | col4 | +------+------+------+------+ | val1 | NULL | 7 | 7 | | val4 | NULL | NULL | abc | | val2 | 2 | 8 | 8 | +------+------+------+------+

Doubts

1. Does SQL correctly converts between integer opreand / column and string operand / column before performing any comparison / mathematical operation? Is there any exception / detail to it?
2. Why second query returned row with col4 having abc ?

PS: I am not talking in the context of any specific database. So, if there is any database specific details / behavior variation to it, please share.

Answer 1

When an integer is compared to a string, the sting is converted to a number.

The number is based on the leading digits of the string. If the string has no digits, then the value is treated as 0 .