4. Understanding the Elements of SQL Statements : SQL Operations : Assignment Operations
 
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Assignment Operations
An assignment operation places a value in a column or variable. Assignment operations occur during the execution of INSERT, UPDATE, FETCH, CREATE TABLE...AS SELECT, and embedded SELECT statements. Assignments can also occur within a database procedure.
When an assignment operation occurs, the data types of the assigned value and the receiving column or variable must either be the same or comparable.
If the data types are not the same, comparable data types are converted.
If the data types are not comparable, convert the assignment value into a type that is the same or comparable with the receiving column or variable.
For information about the type conversion functions, see Default Type Conversion (see Default Type Conversion).