sql alter table not null constraint

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

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 que

Table Sales.orderdetails Order Details Form Sales.orders Order Form Sales.shippers Freight company Table SQL Server Some basic commands: Query Database exists: If db_id ("TestDB") is not null; Check if the

Alter TABLE table1 ALTER COLUMN [name] varchar () NULL; Table1 Table Name Name field name why add [], because name is the SQL keyword will conflict error, so just in case. If the name field has already created an index, you must d

Tags: Main table encountered str no COL efault first left connected Div LINQ to SQL, the LINQ to entity encounters a problem with the primary table, from table to relationship, the primary table has records, and the table may not

SQL WHERE variable IS NULL causes full table scan ProblemsToday, when reviewing the stored procedures of the project, we found that there are a large number of statements to judge whether the variable is NULL in the condition, such:SET @ SQL = 'select * FROM Comment with (no

Create table # staff (id int)Insert into # staff values (9)If exists (select 1 from # staff where id = 9)Print ('pass ')ElsePrint ('fail ')Drop table # staffChange the value of where id = 9 to another value and output fail. You can understand the other values. 1 is not a field of the temporary table # class, but a tag used to judge whether the query is empty. --

Copy Code code as follows: SET @SQL = ' SELECT * Comment with (NOLOCK) WHERE 1=1 and (@ProjectIds is Null or ProjectID = @ProjectIds) and (@Scores is null or Score = @Scores) ' Remember, in the past when doing Oracle development, this writing will lead to a full table scan, not indexed, do not k