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DEFAULT Constraint

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DEFAULT Constraint

The DEFAULT constraint in SQL is used to assign a default value to a column when no value is specified during an INSERT operation.

📝Syntax
CREATE TABLE table_name (
    column_name datatype DEFAULT value
);
default-constraint.sql
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💡What is DEFAULT Constraint?
  • 1Assigns a default value to a column.
  • 2Used when no value is provided.
  • 3Improves data consistency.
  • 4Works during INSERT operations.
💡How DEFAULT Works
  • 1If value is not specified, default is used.
  • 2Database automatically fills the value.
  • 3Works at insert time only.
  • 4Can use constants or functions.
💡Examples of DEFAULT
  • 1Status DEFAULT 'Active'.
  • 2CreatedAt DEFAULT CURRENT_DATE.
  • 3Quantity DEFAULT 1.
  • 4Country DEFAULT "India".
💡Use Cases of DEFAULT
  • 1User registration systems.
  • 2Order management systems.
  • 3Logging and tracking tables.
  • 4Inventory management.
💡Advantages of DEFAULT
  • 1Reduces manual input.
  • 2Ensures consistent data.
  • 3Simplifies INSERT queries.
  • 4Prevents NULL values in optional fields.
💡Limitations of DEFAULT
  • 1Does not apply on UPDATE.
  • 2May hide missing input errors.
  • 3Limited to simple expressions in some DBs.
  • 4Can be overridden explicitly.
🏢Real-world
  • 1Set default user status as Active.
  • 2Automatically assign current date.
  • 3Simplify insert operations.
  • 4Ensure consistent data values.
  • 5Reduce missing field errors.
⚠Common Mistakes
  • 1Using incorrect default data types.
  • 2Assuming default applies on updates.
  • 3Forgetting database-specific functions.
  • 4Overusing default values unnecessarily.
✅Best Practices
  • 1Use meaningful default values.
  • 2Apply defaults for optional fields.
  • 3Combine with NOT NULL when needed.
  • 4Avoid complex expressions as defaults.
✅Quick Summary
  • DEFAULT sets automatic values for columns.
  • Used when no value is provided in INSERT.
  • Improves data consistency.
  • Works only during insertion.
  • Common for status and timestamps.
🎯Interview Questions
Q1. What is DEFAULT constraint?
Answer: It sets a default value for a column when no value is provided.
Q2. When is DEFAULT applied?
Answer: During INSERT operations.
Q3. Does DEFAULT work on UPDATE?
Answer: No, it only works on INSERT.
Q4. Can DEFAULT use functions?
Answer: Yes, like CURRENT_DATE or NOW().
Q5. Why use DEFAULT constraint?
Answer: To simplify inserts and ensure consistent values.
❓Quiz

What does DEFAULT constraint do?

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NOT NULL ConstraintUNIQUE ConstraintCHECK ConstraintAUTO_INCREMENTSequences in SQLSQL FunctionsWhat is SQL?History of SQLFeatures of SQLWhy Learn SQL?
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