MySQL index used: When the field is varchar type, the condition is to be wrapped up with ' '

Conclusion:

When MySQL is in an int type, the search condition where num= ' 111 ' and where num=111 can use the index of the field.
When MySQL field is a varchar type, the search condition where num= ' 111 ' can use the index, where num=111 can not use the index

Verification process:

To build a table statement:

CREATE TABLE ' Gyl ' (  ' id ' int (one) not null auto_increment,  ' str ' varchar (255) is not NULL,  ' num ' int (one) ' NOT NULL Default ' 0 ',  ' obj ' varchar (255) Default NULL,  PRIMARY key (' id '),  key ' str_x ' (' str '),  key ' num_x ' (' num ' ') Engine=innodb  DEFAULT Charset=utf8;

Inserting data into a table using a self-replicating statement

Insert INTO Gyl (' str ', ' num ') VALUES (123123, ' 12313 ');

Insert INTO Gyl (' str ', ' num ') select ' str ', ' num ' from Gyl;

Change data Update Gyl set Num=id,str=id

Results:

Mysql> Explain select * from Gyl where str=123123 limit 1;+----+-------------+-------+------+---------------+------+ ---------+------+--------+-------------+| ID | Select_type | Table | Type | Possible_keys | Key | Key_len | Ref | Rows |  Extra |+----+-------------+-------+------+---------------+------+---------+------+--------+-------------+| 1 | Simple | Gyl | All | str_x | NULL | NULL | NULL | 262756 | Using where |+----+-------------+-------+------+---------------+------+---------+------+--------+-------------+1 Row in setmysql> explain select * from Gyl where str= ' 123123 ' limit 1;+----+-------------+-------+------+------------- --+-------+---------+-------+--------+-------------+| ID | Select_type | Table | Type | Possible_keys | Key | Key_len | Ref | Rows |  Extra |+----+-------------+-------+------+---------------+-------+---------+-------+--------+-------------+| 1 | Simple | Gyl | Ref | str_x | str_x | 257 | ConsT | 131378 | Using where |+----+-------------+-------+------+---------------+-------+---------+-------+--------+------------- +1 row in setmysql> explain select * from Gyl where num= ' 12313 ' limit 1; +----+-------------+-------+------+---------------+-------+---------+-------+--------+-------+| ID | Select_type | Table | Type | Possible_keys | Key | Key_len | Ref | Rows |  Extra |+----+-------------+-------+------+---------------+-------+---------+-------+--------+-------+| 1 | Simple | Gyl | Ref | num_x | num_x | 4 | Const |       131378 | |+----+-------------+-------+------+---------------+-------+---------+-------+--------+-------+1 row in set1065- Query was emptymysql> explain select * from Gyl where num=12313 limit 1;+----+-------------+-------+------+----------- ----+-------+---------+-------+--------+-------+| ID | Select_type | Table | Type | Possible_keys | Key | Key_len | Ref | Rows | Extra |+----+-------------+-------+------+---------------+-------+---------+-------+--------+-------+| 1 | Simple | Gyl | Ref | num_x | num_x | 4 | Const |       131378 | |+----+-------------+-------+------+---------------+-------+---------+-------+--------+-------+1 row in Set

　　

MySQL index used: When the field is varchar type, the condition is to be wrapped up with ' '