MySQL load NULL values from CSV data

问题:

I have a file that can contain from 3 to 4 columns of numerical values which are separated by comma. Empty fields are defined with the exception when they are at the end of the row:

1,2,3,4,5
1,2,3,,5
1,2,3

The following table was created in MySQL:

+-------+--------+------+-----+---------+-------+
| Field | Type   | Null | Key | Default | Extra |
+-------+--------+------+-----+---------+-------+
| one   | int(1) | YES  |     | NULL    |       | 
| two   | int(1) | YES  |     | NULL    |       | 
| three | int(1) | YES  |     | NULL    |       | 
| four  | int(1) | YES  |     | NULL    |       | 
| five  | int(1) | YES  |     | NULL    |       | 
+-------+--------+------+-----+---------+-------+

I am trying to load the data using MySQL LOAD command:

LOAD DATA INFILE \'/tmp/testdata.txt\' INTO TABLE moo FIELDS 
TERMINATED BY \",\" LINES TERMINATED BY \"\\n\";

The resulting table:

+------+------+-------+------+------+
| one  | two  | three | four | five |
+------+------+-------+------+------+
|    1 |    2 |     3 |    4 |    5 | 
|    1 |    2 |     3 |    0 |    5 | 
|    1 |    2 |     3 | NULL | NULL | 
+------+------+-------+------+------+

The problem lies with the fact that when a field is empty in the raw data and is not defined, MySQL for some reason does not use the columns default value (which is NULL) and uses zero. NULL is used correctly when the field is missing alltogether.

Unfortunately, I have to be able to distinguish between NULL and 0 at this stage so any help would be appreciated.

Thanks S.

edit

The output of SHOW WARNINGS:

+---------+------+--------------------------------------------------------+
| Level   | Code | Message                                                |
+---------+------+--------------------------------------------------------+
| Warning | 1366 | Incorrect integer value: \'\' for column \'four\' at row 2 | 
| Warning | 1261 | Row 3 doesn\'t contain data for all columns             | 
| Warning | 1261 | Row 3 doesn\'t contain data for all columns             | 
+---------+------+--------------------------------------------------------+

回答1:

This will do what you want. It reads the fourth field into a local variable, and then sets the actual field value to NULL, if the local variable ends up containing an empty string:

LOAD DATA infile \'/tmp/testdata.txt\'
INTO TABLE moo
fields terminated BY \",\"
lines terminated BY \"\\n\"
(one, two, three, @vfour, five)
SET four = nullif(@vfour,\'\')
;

If they\'re all possibly empty, then you\'d read them all into variables and have multiple SET statements, like this:

LOAD DATA infile \'/tmp/testdata.txt\'
INTO TABLE moo
fields terminated BY \",\"
lines terminated BY \"\\n\"
(@vone, @vtwo, @vthree, @vfour, @vfive)
SET
one = nullif(@vone,\'\'),
two = nullif(@vtwo,\'\'),
three = nullif(@vthree,\'\'),
four = nullif(@vfour,\'\')
;

回答2:

MySQL manual says:

When reading data with LOAD DATA INFILE, empty or missing columns are updated with \'\'. If you want a NULL value in a column, you should use \\N in the data file. The literal word “NULL” may also be used under some circumstances.

So you need to replace the blanks with \\N like this:

1,2,3,4,5
1,2,3,\\N,5
1,2,3

回答3:

The behaviour is different depending upon the database configuration. In the strict mode this would throw an error else a warning. Following query may be used for identifying the database configuration.

mysql> show variables like \'sql_mode\';

回答4:

Preprocess your input CSV to replace blank entries with \\N.

Attempt at a regex: s/,,/,\\n,/g and s/,$/,\\N/g

Good luck.