sql field not null

1 Introduction to null valuesNULL is a unique data type in the database. If a column in a record is NULL, the value of this column is unknown and uncertain. Since it is unknown, there are several possibilities. Therefore, NULL is not a definite

The environment of the program is VB6 + ADO + Access, when you query with SQL statements, you want to combine two fields into one field, but one of the null values in one field directly causes two fields to merge and become null values. Before can

If there is only one table in the caseTable A:ID num1,num21 10 52 10 03 20 0Select ID, num1,num2,num1-num2 as subnum from A;In this case:The results are as follows:ID NUM1 num2 Subnum1 10 5 52 10 0 103 20 0 20In a single-table case, so that in SQL,

Determines whether a variable is null: IF (@VaribleName is NULL) Select a record for which the field value is null: WHERE column_name is NULL ISNULL () function: ISNULL (@VaribleName, ' defaultvalue ') ISNULL (column_name, ' Default Value ')

In fact, there is nothing to say about this problem, today's optimization encountered an SQL statement, because it is more interesting, so I intercepted, simplified the SQL statement, to show you, as shown belowDECLARE @bamboo_Code varchar (3);Set

SQL NOT NULL constraint syntax and NOT NULL instance If we create a table CREATE TABLE AA(id_p int not NULL,LName varchar () not NULL,FName varchar (25),Ass varchar (25),C varchar (205)) The NOT NULL constraint enforces that the column does not

When a field in the database is set to Null, the mobile phone will return Null by default. if I want the mobile phone to return Null, I have to make a judgment. 1. if I want to leave the returned result blank on the mobile phone, I will not make any

Today, my classmate asked me a question, I think it is very interesting, now recorded down to discuss. The problem is: in a table, there is a field that is allowed to be empty, and null can be repeated, but a value that is not NULL needs to be

MySQL field constraints: null, notnull, default, auto_increment bitsCN.com MySQL field constraints: null, not null, default, auto_increment In MySQL, each field definition contains additional constraints or modifiers, which can be used to add

The introduction of 1null value NULL is a unique data type in the database. When a column in a record is NULL, the value of this column is unknown and uncertain. Since it is unknown, there are several possibilities. Therefore, NULL is not a definite