PostgreSQL Control Flow Statements

Control flow statements in PostgreSQL allow you to control the execution sequence of SQL statements based on specific conditions. They are essential for writing efficient functions, procedures, and queries.

1?? IF Statement

? What is an IF Statement?

The IF statement executes certain actions based on whether a specified condition is TRUE or FALSE.

• It is widely used in stored procedures and functions to control execution flow.

A stored procedure is a named set of SQL statements that are stored in the database server. It can have input parameters, output parameters, and a return value.

?? Syntax

IF condition THEN
    -- statements to execute if condition is true
ELSE
    -- statements to execute if condition is false
END IF;
        

?? Key Terms

• condition: A logical expression returning TRUE or FALSE.
• THEN: Marks the beginning of statements executed if the condition is true.
• ELSE: (Optional) Executes alternative statements if the condition is false.
• END IF: Marks the end of the IF block.

?? Common Errors & Solutions

• Syntax Errors: Ensure correct placement of THEN, ELSE, and END IF.
• Null Conditions: Handle NULL values properly to avoid unexpected results.

?? Key Points

1. Keep conditions simple for better readability.
2. Use CASE for multiple conditions.
3. Comment complex logic for maintainability.
4. Consider performance when using IF in large datasets.

Sample Query

DO $$
BEGIN
    IF (SELECT COUNT(*) FROM users) > 1000 THEN
        RAISE NOTICE 'Large dataset';
    ELSE
        RAISE NOTICE 'Small dataset';
    END IF;
END $$;
        

?Common Q&A

• How do IF statements impact execution time on large datasets?

Overuse of IF inside loops or functions can slow down execution. Use bulk operations and indexing to optimize performance.

• What are BEGIN and END keywords?

BEGIN and END define a block of procedural code in Postgres functions.

2?? CASE Statement

? What is a CASE Statement?

The CASE statement allows decision-making within queries, making workflows more efficient.

?? Syntax

Simple CASE Statement

CASE search-expression
   WHEN expression_1 THEN result_1
   WHEN expression_2 THEN result_2
   ELSE default_result
END;
        

Searched CASE Statement

CASE
    WHEN condition_1 THEN result_1
    WHEN condition_2 THEN result_2
    ELSE default_result
END;
        

?? Key Terms

• search-expression: The value being compared.
• WHEN: Specifies conditions to match.
• THEN: Defines what to return or execute.
• ELSE: (Optional) Default action if no match is found.

?? Common Errors & Solutions

• CASE_NOT_FOUND Exception: Ensure an ELSE clause is present.

?? Key Points

1. Use Simple CASE for equality comparisons.
2. Use Searched CASE for complex conditions.
3. Improves query readability and efficiency.

Sample Query

SELECT employee_name,
       CASE
           WHEN salary > 50000 THEN 'High'
           WHEN salary BETWEEN 30000 AND 50000 THEN 'Medium'
           ELSE 'Low'
       END AS salary_category
FROM employees;
        

?Common Q&A

• What errors can occur and how to solve them?

CASE_NOT_FOUND Error: Occurs if no ELSE clause is provided and no match is found. Solution: Always include an ELSE clause.

Data Type Mismatch: Ensure all return values have the same data type.

3?? LOOP Statements

? What is a LOOP?

A LOOP executes a block of code repeatedly until an EXIT or RETURN statement is encountered.

?? Syntax

<<label>>
LOOP
    statements;
    EXIT WHEN condition;
END LOOP;        

?? Key Terms

• label: Identifies the loop.
• EXIT WHEN: Terminates the loop when a condition is met.
• DECLARE: Used to define variables in Postgres.
• DO: Executes an anonymous code block without creating a function.
• RAISE NOTICE '%': Prints a message for debugging.

?? Common Errors & Solutions

1. Define termination conditions to avoid infinite loops.

?? Key Points

1. Use increment/decrement operations for controlled execution.
2. Nested loops should be structured carefully.

Sample Query

DO $$
DECLARE counter INT := 1;
BEGIN
    <<main_loop>>
    LOOP
        RAISE NOTICE 'Counter: %', counter;
        counter := counter + 1;
        EXIT WHEN counter > 5;
    END LOOP;
END $$;        

?Common Q&A

• How does EXIT work in nested loops?

EXIT exits only the loop where it is mentioned unless a label is used to exit an outer loop.

• When to use := & = operators in Postgres?

It is used for variable assignment in Postgres, whereas = is used for comparison in queries.

• What is an unconditional loop?

A loop that runs indefinitely unless explicitly exited. Example is :

LOOP   
    EXIT WHEN counter > 10;   
END LOOP;        

4?? WHILE Loop

? What is a WHILE Loop?

A WHILE loop repeats a block of code as long as a specified condition holds TRUE.

?? Syntax

WHILE condition LOOP
    statements;
END LOOP;        

?? Common Errors & Solutions

1. Avoid infinite loops by ensuring condition termination.

?? Key Points

1. Useful for data migrations, batch processing, and bulk updates.
2. Pretest loop: condition is checked before execution.

Sample Query

DO $$
DECLARE counter INT := 1;
BEGIN
    WHILE counter <= 5 LOOP
        RAISE NOTICE 'Counter: %', counter;
        counter := counter + 1;
    END LOOP;
END $$;        

5?? EXIT Statement

? What is EXIT?

The EXIT statement is used to terminate loops and blocks.

?? Syntax

EXIT [label] [WHEN condition];        

?? Key Points

1. Can be used in any loop type (LOOP, WHILE, FOR).
2. Without a condition, it exits immediately.
3. Labels help exit specific loops in nested structures.

Sample Query

DO $$
DECLARE counter INT := 1;
BEGIN
    <<outer_loop>>
    LOOP
        RAISE NOTICE 'Counter: %', counter;
        counter := counter + 1;
        EXIT WHEN counter > 3;
    END LOOP;
END $$;        

?Common Q&A

• Can EXIT be used to exit from an inner loop or the entire nested loop?

By default, EXIT exits only the current loop. Use labels to exit an outer loop.

6?? CONTINUE Statement

? What is CONTINUE?

The CONTINUE statement skips the remaining statements in the current iteration and moves to the next iteration.

?? Syntax

CONTINUE [ label ] [ WHEN condition ];        

?? Key Points

1. Helps manage loop flow efficiently.
2. Skips specific iterations without terminating the loop.
3. Labels specify which loop to continue in nested structures.

Sample Query

DO $$
DECLARE counter INT := 0;
BEGIN
    LOOP
        counter := counter + 1;
        IF counter = 3 THEN
            CONTINUE;
        END IF;
        RAISE NOTICE 'Counter: %', counter;
        EXIT WHEN counter >= 5;
    END LOOP;
END $$;        

?Common Q&A

• What is Labels in WHILE, EXIT, LOOP, CONTINUE ?

A label is an identifier that helps control execution in nested loops.

With label :

<<outer_loop>> 
 LOOP 
       <<inner_loop>>  
        LOOP   
             EXIT outer_loop WHEN some_condition;  
        END LOOP;  -- For inner loop
END LOOP; -- for outer loop        

Without label :

LOOP
   EXIT WHEN some_condition; 
END LOOP;        

?? Conclusion

Control flow statements in PostgreSQL provide powerful mechanisms to manage query execution, improve efficiency, and create dynamic procedures. Mastering these concepts helps in writing optimized SQL logic. ??

Which of these PostgreSQL control flow statements do you use most often? If you have any question about any of the above topics. Let me know in the comments! below??.

HAVE A GREAT DAY AHEAD ?? ALL THE BEST