MySQL, better to insert NULL or empty string

MySQLsqlsql-null

I have a form on a website which has a lot of different fields. Some of the fields are optional while some are mandatory. In my DB I have a table which holds all these values, is it better practice to insert a NULL value or an empty string into the DB columns where the user didn't put any data?

Best Answer

By using NULL you can distinguish between "put no data" and "put empty data".

Some more differences:

A LENGTH of NULL is NULL, a LENGTH of an empty string is 0.
NULLs are sorted before the empty strings.
COUNT(message) will count empty strings but not NULLs
You can search for an empty string using a bound variable but not for a NULL. This query:
```
SELECT  *
FROM    mytable 
WHERE   mytext = ?
```
will never match a NULL in mytext, whatever value you pass from the client. To match NULLs, you'll have to use other query:
```
SELECT  *
FROM    mytable 
WHERE   mytext IS NULL
```

Related Solutions

Sql – Insert into … values ( SELECT … FROM … )

Try:

INSERT INTO table1 ( column1 )
SELECT  col1
FROM    table2

This is standard ANSI SQL and should work on any DBMS

It definitely works for:

Oracle
MS SQL Server
MySQL
Postgres
SQLite v3
Teradata
DB2
Sybase
Vertica
HSQLDB
H2
AWS RedShift
SAP HANA
Google Spanner

Mysql – concatenate multiple MySQL rows into one field

You can use GROUP_CONCAT:

SELECT person_id,
   GROUP_CONCAT(hobbies SEPARATOR ', ')
FROM peoples_hobbies
GROUP BY person_id;

As Ludwig stated in his comment, you can add the DISTINCT operator to avoid duplicates:

SELECT person_id,
   GROUP_CONCAT(DISTINCT hobbies SEPARATOR ', ')
FROM peoples_hobbies
GROUP BY person_id;

As Jan stated in their comment, you can also sort the values before imploding it using ORDER BY:

SELECT person_id, 
       GROUP_CONCAT(hobbies ORDER BY hobbies ASC SEPARATOR ', ')
FROM peoples_hobbies
GROUP BY person_id;

As Dag stated in his comment, there is a 1024 byte limit on the result. To solve this, run this query before your query:

SET group_concat_max_len = 2048;

Of course, you can change 2048 according to your needs. To calculate and assign the value:

SET group_concat_max_len = CAST(
                     (SELECT SUM(LENGTH(hobbies)) + COUNT(*) * LENGTH(', ')
                           FROM peoples_hobbies
                           GROUP BY person_id) AS UNSIGNED);

Best Answer

Related Solutions

Sql – Insert into … values ( SELECT … FROM … )

Mysql – concatenate multiple MySQL rows into one field

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