SQL Constraints
Constraints can be specified when the table is created (inside the CREATE TABLE statement) or after the table is created (inside the ALTER TABLE statement). Ensures that a column (or combination of two or more columns) have an unique identity which helps to find a particular record in a table more easily and quickly FOREIGN KEY - Ensure the referential integrity of the data in one table to match values in another table CHECK - Ensures that the value in a column meets a specific condition DEFAULT - Specifies a default value when specified none for this column The next chapters will describe each constraint in detail
sql server - foreign key constraint violation issue - Database Administrators Stack Exchange
If I could not delete a new student with no enrollments (and no GPA entry) then I would understand the constraint violation, but I can delete that student. So it sets about deleting the enrollments, which cause a trigger to fire which creates the GPA entry, which then causes the constraint violation? So the solution is to create a GPA entry when I create a student
MySQL :: MySQL 5.0 Reference Manual :: 13.1.4 ALTER TABLE Syntax
Updates and writes to the table that begin after the ALTER TABLE operation begins are stalled until the new table is ready, then are automatically redirected to the new table without any failed updates. If you use any option to ALTER TABLE other than RENAME, MySQL always creates a temporary table, even if the data wouldn't strictly need to be copied (such as when you change the name of a column)
Alter Table Drop Column
Name: Email: Comment: Home Tuning Services Featured Articles How to cache stored procedure results using a hash key There are a lot of different design patterns that lend themselves to creating SQL Server Database Optimization Guide In the troubleshooting guide we went over the different physical bottlenecks that can Yet Another Temp Tables Vs Table Variables Article The debate whether to use temp tables or table variables is an old Using Union Instead of OR Sometimes slow queries can be rectified by changing the query around a bit. We obfuscate data as it is moved to another environment and rather than updating the data it was actually much quicker to drop the column and add it back along with a default value
SQL Server Constraints
General Concepts A constraint is a property assigned to a column or the set of columns in a table that prevents certain types of inconsistent data values from being placed in the column(s). For example, in the previous versions of SQL Server if you wanted to do a cascade delete from the referenced table when the appropriate record in the parent table is deleted, you had to create a trigger which executed on delete of the parent table, but now you can simply specify the ON DELETE clause in the REFERENCES clause
FOR EACH ROW triggers are called once for each inserted, updated, or deleted row.QUEUE is implemented for syntax compatibility with HSQL and has no effect.The trigger need to be created in the same schema as the table. Currently, enabling this is experimental only.This is a global setting, which means it is not possible to open multiple databases with different modes at the same time in the same virtual machine
ALTER TABLE only allows columns to be added that can contain nulls, or have a DEFAULT definition specified, or the column being added is an identity or timestamp column, or alternatively if none of the previous conditions are satisfied the table must be empty to allow addition of this column. Fill in your details below or click an icon to log in: Email (required) (Address never made public) Name (required) Website You are commenting using your WordPress.com account
MySQL :: MySQL 5.6 Reference Manual :: 13.1.7 ALTER TABLE Syntax
These keywords are deprecated beginning with MySQL Cluster NDB 7.3; they continue to be supported in MySQL Cluster NDB 7.4, but are subject to removal in a future version of MySQL Cluster. Updates and writes to the table that begin after the ALTER TABLE operation begins are stalled until the new table is ready, then are automatically redirected to the new table without any failed updates
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sql server 2008 - Quickly change NULL column to NOT NULL - Database Administrators Stack Exchange
One problem with this is that even if you know the column in fact has no NULLs the constraint is not trusted by the query optimiser which means that the plans can be sub optimal. Adding the constraint with NOCHECK means that no rows will need to be read to verify it, and if you are starting from a position where the column does not contain NULL values (and if you know none will be added between checking and adding the constraint) then, as the constraint prevents NULL values being created from future INSERT or UPDATE operations, this will work
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