sql not null constraint

Tags: check int enable LTE delete nbsp Add ons tableTo add a constraint to an existing data table, but not to use the constraint to existing data, use the WITH NOCHECK clause when executing the ALTER TABLE statement, so that the new constraint will not constrain the existing data, but only the new data will be constrained to check:ALTER TABLE [TableName] with Noc

a group.Code --drop the unique constraintALTER TABLE [dbo].[test] DROP CONSTRAINT [UQ__test__737584F63A81B327]--insert two null valuesinsert into test values(NULL,12),(NULL,13)select * from testselect Name,SUM(age)as sumage from test group by Name We can see that null is di

function is useful if null values can lead to error results. Remember that when you use an asterisk, the Count function calculates all the rows. The following example shows the effect of NULL values in AVG and Count aggregate functions: Copy Code code as follows: SET NOCOUNT on Go CREATE TABLE Xcount (Pkey1 INT IDENTITY not NULL

One, add constraint (cascade delete)1, CREATE TABLE structure when adding create table Userdetails(ID int identity (primary key),Name varchar () not NULL,--real nameUserId int,FOREIGN KEY (userid) references UserInfo (userid) on DELETE cascade2. Add ALTER TABLE [dbo] when the table structure already exists.[Api_user_label]With CHECK Add CONSTRAINT [Fk_api_user_la

Tags: class str conflict resolution SRC Technology blog Eid demoIf you add a constraint to the table where the data exists, it is possible that the data does not conform to the CHECK constraint and the add constraint fails.Such as:This is a table that adds a check constraint for the ID number.Use demo--points to the da

First step: Delete the original foreign KEY constraintALTER TABLE child_table DROP FOREIGN KEY ' fk_reference_2 ';Step Two: Add a new foreign KEY constraint and increase the on DELETE CASCADEALTER TABLE child_table ADD CONSTRAINT ' fk_reference_2 ' FOREIGN KEY (' parent_id ') REFERENCES parent_table (' parent_id ') On DELETE CASCADE on UPDATE RESTRICT;In this way, it is possible to delete the parent n

restricts the length of the user name, it cannot cause the database to be truncated, and the condition is not exploited. The SQL statement that the login verifies must be authenticated together with the user name and password . If the verification process is based on the user name to find the corresponding password, and then compared to the password, then can not be exploited. Because when using dumb to query the password for the user name, the d

Label:The null value does not satisfy any lookup criteria other than is [not] null. – If NULL participates in arithmetic operations, the value of the arithmetic expression is null.– If NULL participates in the comparison operation, the result can be treated as false. can be

SQL Server Replace Null:isnull (Arg,value) For example: Select IsNull (price,0.0) from orders, if Price is null, replace with 0.0 Compared to null: is not null,is null For example, select * from to orders where price is null a

Here are some of the information found online to solve this problem:Get this job_id first:USE [msdb]GO/****** Object: Job [dbbackup_diff.Subplan_1] Script Date: 04/08/2015 10:22:46 ******/@job_id=N‘588acfb8-630b-41c3-aa47-90d3671bda90‘@delete_unused_schedule=1GOAccording to job_id to subplan_id:select subplan_id FROM msdb.dbo.sysmaintplan_subplans where job_id=‘588acfb8-630b-41c3-aa47-90d3671bda90‘subplan_idFE56B397-4704-4AA7-BE49-767C1574E7D0Remove subplan_id from system tables Msdb.dbo.sys

of a multiplication of two table records is a Cartesian product query, if the left table has n records, the right table has m records, Cartesian product query has N*M records, which often contain a lot of wrong data, so this query method is not commonly used.SELECT * from a b where a.id = b.ID;or select * from a INNER join B on a.id=b.id; Internal connection query: The query is the left table and the right table can find records of corresponding records.SELECT * from a LEFT join B on a.id=b.id;

specific principle here is not clear, the above is only speculation, hereby recordedIf you find a reason to update!Update:Https://stackoverflow.com/questions/22013511/mysql-error-1215-hy000-cannot-add-foreign-key-constraintThat is to say, my above guess is correct!Take a look at the unique test:Transferred from: http://www.cnblogs.com/Andy963/p/7634537.htmlThe last thing to note is that the foreign key must be the primary key or the unique key of the other table (that is, either set the column

[ORG/springframework/jdbc/support/sql-error-codes.xml]Info [org. Spring framework. JDBC. Support. sqlerrorcodesfactory]-sqlerrorcodes loaded: [DB2, Derby, H2, hsql, Informix, MS-SQL, MySQL, Oracle, PostgreSQL, Sybase]Org. springframework. JDBC. uncategorizedsqlexception: sqlmapclient operation; uncategorized sqlexception for SQL [];

What if SQL fuzzy query encounters a null value ?, SQL fuzzy query is blank Author: iamlaosong The SQL query statement uses % for fuzzy query. in the program, users are generally required to enter part of the information and perform fuzzy query based on the information. For example, if you enter 340104, the following s

2601, Level 14, State 1, line 1thYou cannot insert a row of duplicate keys in an object ' DBO.TEST_TB ' with a unique index ' UN_TEST_TB '.The statement was terminated. So the solution is not going to work. Solution 2: Add a constraint so that SQL server, when inserting data, verifies that there is a value in the existing data that is now being inserted. Since this

server| Policy data integrity is a key point for any database system to ensure. Regardless of how well the system is planned, the problem of NULL data values always exists. This article explores the 3 issues involved in handling these values in SQL Server: counting, using null table values, and foreign key processing. Handling

The table structure is created like the following code Copy Code code as follows: CREATE TABLE TEST_TB ( TestID int NOT null identity (1,1) primary key, Caption nvarchar (MB) null ); Go Solution 1: The first idea for this question may be: Is it OK to add a unique key to the caption field? OK, let's follow this thread and create a unique index first. Copy Code code a

groupedSuch as:-The total number of girls and boys receivedSelect Sex,count (*) from Student GROUP by sexOrder of Query statements:Select from where the group by has an order byNote: Where is the filter for the source data. It can only use columns that are referred to in the table following the fromAn aggregate function cannot be used after a where condition, and an error will be made if usedHavingIf you are filtering the result set after grouping, then you need to have a having, because the wh

wrong." But in relational theory, it's not that simple. There is a situation in which there is no absolute right or wrong: NULL. This is the three-valued logic of relational theory. There are also some common null-related situations. For example, statistical functions (which are also referred to as aggregate functions, set functions) usually ignore null. Howeve

The following code creates a table structure: Copy codeThe Code is as follows: create table test_tb ( TestId int not null identity (1, 1) primary key, Caption nvarchar (100) null ); GO Solution 1:For this question, the first thought may be: Can I add a unique key to the Caption field? Well, let's proceed with this idea and create a unique index first.Copy codeThe Code is as follows: create unique noncluste