This form sets or resets per-attribute options. SET STATISTICS acquires a SHARE UPDATE EXCLUSIVE lock. For more information on the use of statistics by the PostgreSQL query planner, refer to Section 14.2. The target can be set in the range 0 to 10000 alternatively, set it to -1 to revert to using the system default statistics target ( default_statistics_target). This form sets the per-column statistics-gathering target for subsequent ANALYZE operations. You can only use SET NOT NULL when the column contains no null values. These forms change whether a column is marked to allow null values or to reject null values. Default values only apply in subsequent INSERT or UPDATE commands they do not cause rows already in the table to change. These forms set or remove the default value for a column. A USING clause must be provided if there is no implicit or assignment cast from old to new type. The optional USING clause specifies how to compute the new column value from the old if omitted, the default conversion is the same as an assignment cast from old data type to new. The optional COLLATE clause specifies a collation for the new column if omitted, the collation is the default for the new column type. Indexes and simple table constraints involving the column will be automatically converted to use the new column type by reparsing the originally supplied expression. This form changes the type of a column of a table. IF EXISTSĭo not throw an error if the table does not exist. If IF EXISTS is specified and the column does not exist, no error is thrown. You will need to say CASCADE if anything outside the table depends on the column, for example, foreign key references or views. Indexes and table constraints involving the column will be automatically dropped as well. This form adds a new column to the table, using the same syntax as CREATE TABLE. RENAME CONSTRAINT constraint_name TO new_constraint_nameĪLTER TABLE ALL IN TABLESPACE name ]ĪDD column_name data_type ]ĭROP column_name ĪLTER column_name TYPE data_type ĪLTER column_name SET DEFAULT expressionĪLTER column_name DROP DEFAULTĪLTER column_name USING INDEX index_name SQL Server - GRANT/REVOKE Permissions to User.Synopsis ALTER TABLE name ĪLTER TABLE name Now, if you try to insert or update a record in the Employee table where Salary is not within the specified range, then it will raise the following error. The INSERT statement conflicted with the CHECK constraint "CHK_Emp_Salary". The conflict occurred in database "HR", table "dbo.Employee", column 'Salary'.Ī check constraint can be created with any logical Boolean that returns True or False based on logical operators.įor example, a boolean expression Salary > 2000 and Salary < 4000 specifies that the value in the Salary column must be more than 2000 and less than 4000.Ī boolean expression of check constraint can use multiple columns.Ī single column can have multiple check constraints.Ĭheck constraints reject values that evaluate to False. If you try to insert a value outside the check constraint value range then SQL engine throws an error stating that the insert statement conflicted with the CHECK constraint and the statement was terminated.Ī check constraint does not evaluate a NULL value. So NULL can be inserted without restriction. ![]() Use NOT NULL constraint to restrict the NULL values.Ĭheck constraints are not validated during the Delete statements. You can enable or disable check constraints instead of deleting them.
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