mysql alter column not null

many tables that contain nullable columns (Nullable column), just because it is the default option. Unless you really want to save NULL, the column is defined as NOT NULL. MySQL makes it difficult to optimize queries that reference nullable columns, which make indexes, in

); Mysql> insert into my_table (phone) VALUES (""); The two statements insert values into the phone column, but the first statement inserts a NULL value and the second statement inserts an empty string. The first one can be thought of as "the phone number is unknown", while the second one can mean "she has no phone number ". In SQL, the

Mysql basics: mysql column type -- string bitsCN.com Mysql basics: mysql column type-string · CHAR This is a synonym for CHAR (1. · [NATIONAL] VARCHAR (M) [BINARY] Variable-length string. M indicates the maximum

columns containing null according to your own wishes, you can perform the preceding operations. [MySQL ]:MySQL is the same as sqlserver, and null is the minimum by default. The solution is the same as that of sqlserver. Note:1. When a null

MySQL NULL value processing instance details, mysqlnull MySQL NULL value processing We already know that MySQL uses the SQL SELECT command and the WHERE clause to read data in the data table. However, when the provided query condition field is

data type is int), Date (its data type is timestamp). Now insert a record into this table, where a null null value is inserted into the Date field. But when we query, the result is the current time that the record was inserted. What is this all about? In fact, this is a trap that is often encountered when executing SQL statements in a MySQL database:

From: http://lz12366.iteye.com/blog/673290 The concept of null value is a common cause of confusion among SQL beginners. They often think that null is the same as an empty string. No! For example, the following statements are completely different: Mysql> insert into my_table (phone) values (null );

SQL statements in a MySQL database: Null values are not necessarily empty. When the operation, obviously inserted is a null value of data, but the last query is not a null value. In a MySQL database, NULL is a special type of

First take a look at the simplest example, in test, add a field, the field name is birth, the type is date type. Mysql> ALTER TABLE test add column Birth date;Query OK, 0 rows affected (0.36 sec)records:0 duplicates:0 warnings:0 Check the data and see the results: Mysql> select * from test;+------+--------+------------

We have seen the SQL SELECT command using the WHERE clause to fetch data from the MySQL table. However, when we try to give the condition to compare the value of the field or column to NULL, it does not work correctly. To handle this situation, MySQL provides three major operators is

Usage of the is null statement in the mysql database Note that in mysql, 0 or null means false while other values mean true. The default true value of Boolean operations is 1. The special processing of null is in the previous chapter. To determine which animal is no longer a

Usage of the is null statement in the mysql database Note that in mysql, 0 or null means false while other values mean true. The default true value of Boolean operations is 1. The special processing of null is in the previous chapter. To determine which animal is no longer a

, TownidSMALLINT not NULL, PRIMARY KEY(Peopleid));Here is the data we inserted into this people table:The data fragment contains four people named "Mikes" (two of them Sullivans, two surname McConnells), two are 17 years of age, and a different name for Joe Smith. The primary purpose of this table is to return the corresponding Peopleid based on the specified user name, first name, and age. For example, we may need to find the Peopleid of a user na

the calculation.Defines a column as not NULL. This process is faster and requires less space. It can also sometimes simplify the query because there is no need to check for exception null.Consider using the enum column. If you have a column that contains only a limited number of specific values, you should consider co

Explain can analyze the execution of the select statement, that is, the "Execution Plan" of MySQL. Type Column MySQL finds the required rows in the table. Including (from left to right, from worst to best): | All | index | range | ref | eq_ref | const, system | null | ALL full table scan.

1. NULL is allowed (null/not null)Specifies whether the value of a field can be null. The default is to be empty.At this point, insert the value A = ten, B is empty, and the discovery can be inserted correctly:If you try to insert a value of B = 11, and a does not handle it, the discovery cannot be inserted correctly,

Mysql versions later than 5.0.17 Mysql versions later than 5.0.17 deal with ''and null After MySQL is upgraded to 5.0.17, execute the SQL statement Insert INTO 'table name' () VALUES (); Error: #1264-Out of range value for column ''at row 1 Cause: The new

necessarily set to null. During the operation, the data is clearly inserted with a null value, but the final query result is not a null value. In MySQL databases, NULL represents a special meaning for some special types of columns, not just a

We have seen the SQL SELECT command using the WHERE clause to fetch data from the MySQL table. However, when we try to give the condition to compare the value of the field or column to NULL, it does not work correctly. To handle this situation, MySQL provides three major operators is

timestamp and date) are better than string storage. 2> use an integer to save the IP address. 3. Avoid null as much as possible-if you plan to index a column, avoid setting the column to null as much as possible and define the field as not null. -- You can place a default v