what null

What is the function of not NULL in the new field when the statement ' AAA ' varchar (one) is not null? What is the function of not NULL in the new field when the statement ' AAA ' varchar (one) is not null? is not null possible? ------to solve the

Server Version:5.5.33-31.1-log Percona Server (GPL), Release rel31.1, Revision 566mysql> CREATE TABLE ' T1 ' (' ID ' int (one) not NULL DEFAULT ' 1 ',' NAME ' varchar (ten) not NULL DEFAULT ' ',' Create_time ' timestamp not NULL DEFAULT ' 0000-00-00

Why do you want to set the field to NOT null?1, empty value is not space-consuming2. The null in MySQL is actually occupied space, the following is the explanation from the official MySQL "NULL columns require additional space in the row to

Four myths about indexing using indexes: Effects of null values on indexes Let's do some test data first: Sql> CREATE table t (x int, y int); Table created Note that here I do a unique (federated) index of table T: Sql> Create unique index t_

Is not null and! in Oracle SQL! = NULL: is not null and! = NULL. We often encounter database conditions! = NULL, but the returned result is a NULL set. In fact, the difference is not fully understood. By default, it IS recommended to use is not null

I always thought that a NULL string is NULL. When I inserted a NULL String, I could use NULL. However, some time ago, I found that the NULL Character Set was not included after I used the not in value, at that time, I also thought that it should be

The database does not have the following values: 1. null if it seems that the cell is blank, It is the null value.2.0 occasionally, some cells are '0' by default. The value 0 may be char or Int, so it is divided into 0 and '0' (that is, single

What is a null value (NULL)? A null value, like its name, indicates that a variable has no value. A null value (NULL) is not case-sensitive, that is, NULL is the same as null or NULL. There are three scenarios in which null values are given in PHP:

In PHP because it is a weakly typed language, the different types of values can be implicitly converted, making false,null, ", 0, ' 0′ The comparison of these values is somewhat confusing, now summarizes:Equal judgment' = = NULL = = 0 = = False

Although proficient in the SQL of the people do not have any questions about NULL, but summed up a very full article is still very difficult to find, see an English version, feel good. Tony Hoare invented the null reference in 1965 and considered

The judgment variable is null and is used in many occasions. at the same time, you and many new users often make some mistakes, so I sorted out some common PHP methods to judge whether the variables are empty. 1. isset function: determines whether a

For details about NULL values in Oracle, NULL indicates UNKNOW (unknown) and does not represent any value. For example, if a column in a row does not have any value, it is NULL. For details about NULL values in Oracle, NULL indicates UNKNOW (unknown)

  [Sqlserver ]: Sqlserver considers null to be the smallest. Sort in ascending order: The null value is ranked first by default. Order by case when col is null then 1 else 0 end, Col Sort in descending order: The null value is ranked last by

When we don't know what the data is, it is unknown, it can be null, we call it null, and in Oracle, the table column with null values is zero in length.Oracle allows fields of any one data type to be empty, except for the following two scenarios:1,

You can use php setcookie to set or delete a cookie in php. If you set the cookie, you can set a value. If you delete the cookie, you can delete it if it is null or if it expires, here are some examples. For a long time, when deleting cookies in

Determine the relationship between 0 and ' ' and empty null false start// if (' safdasefasefasf ' ==0) { echo "The string is converted to a number equal to 0"; }//output: The string is converted to a number equal to zero. This is

Handling of NULL or null values for stringin the development process, the author often encountered the following errors in use:1, Error usage one: if (name = = "") { //do something } 2, Error usage two: if (Name.equals ("")) {

Original: null value problem in SQL ServerSQL uses three-valued predicate logic, so the result returned by a logical expression can be true, false, or unknown, and returning true in the three-value logic is not exactly the same as not returning

What does null mean?Null does not represent an empty string, but does not represent 0. What does null mean?Null indicates that you do not know what it is, that is, the original meaning of null is that you do not know what it is, indicating that

Null Value Null indicates that the value is unknown. The null value is different from the blank or zero value. There are no two equal null values. Compare two null values or compare null values with any other values to return unknown results,