Postgres syntax error at or near "IF"

Understanding and Resolving 'syntax error at or near "IF"' in PostgreSQL

Learn why you encounter 'syntax error at or near "IF"' in PostgreSQL and how to correctly implement conditional logic using SQL standards, PL/pgSQL, and CASE statements.

The error message syntax error at or near "IF" is a common stumbling block for developers new to PostgreSQL, especially those coming from other SQL dialects or programming languages. This error typically arises because the IF statement, as a standalone control flow construct, is not part of standard SQL for direct use in queries. Instead, IF is primarily a procedural language construct, most notably found in PostgreSQL's native procedural language, PL/pgSQL, or in other SQL dialects like MySQL's stored procedures.

Why 'IF' is a Syntax Error in Standard SQL Queries

Standard SQL, and by extension PostgreSQL's SQL query language, is declarative. This means you describe what data you want, not how to get it step-by-step. Procedural IF statements, which dictate execution flow based on conditions, belong to procedural extensions of SQL. When you try to use IF directly within a SELECT, INSERT, UPDATE, or DELETE statement, the SQL parser doesn't recognize it as a valid keyword for that context, leading to a syntax error.

flowchart TD
    A[User writes SQL query] --> B{Contains 'IF' statement?}
    B -->|Yes| C[SQL Parser encounters 'IF']
    C --> D{Is it within PL/pgSQL block?}
    D -->|No| E["syntax error at or near 'IF'"]
    D -->|Yes| F[PL/pgSQL handles 'IF' correctly]
    B -->|No| G[Query processed normally]

Flowchart illustrating how PostgreSQL handles the 'IF' keyword in different contexts.

Correct Approaches for Conditional Logic in PostgreSQL

PostgreSQL offers several robust ways to implement conditional logic, depending on whether you need it within a single query or as part of a more complex procedural block.

1. Using the CASE Expression for Query-Level Conditions

The CASE expression is the standard SQL way to implement IF-THEN-ELSE logic directly within SELECT, WHERE, ORDER BY, and other clauses. It evaluates conditions and returns a result based on the first true condition. There are two forms: simple CASE and searched CASE.

-- Searched CASE expression
SELECT
    product_name,
    price,
    CASE
        WHEN price > 100 THEN 'Expensive'
        WHEN price BETWEEN 50 AND 100 THEN 'Moderate'
        ELSE 'Affordable'
    END AS price_category
FROM
    products;

-- Simple CASE expression (for checking equality against a single expression)
SELECT
    order_id,
    status,
    CASE status
        WHEN 'pending' THEN 'Waiting for payment'
        WHEN 'shipped' THEN 'On its way'
        WHEN 'delivered' THEN 'Completed'
        ELSE 'Unknown Status'
    END AS order_description
FROM
    orders;

Examples of CASE expressions for conditional logic within a SELECT statement.

2. Using PL/pgSQL for Procedural Logic (Functions, Triggers)

When you need more complex, multi-statement conditional logic, loops, or variable assignments, you should use PL/pgSQL. This is PostgreSQL's procedural language, and it's where the IF-THEN-ELSE statement is fully supported. You'll typically use PL/pgSQL within functions, stored procedures, or triggers.

CREATE OR REPLACE FUNCTION get_product_status(product_id INT)
RETURNS TEXT AS $$
DECLARE
    current_stock INT;
    status_text TEXT;
BEGIN
    SELECT stock_quantity INTO current_stock FROM products WHERE id = product_id;

    IF current_stock IS NULL THEN
        status_text := 'Product Not Found';
    ELSIF current_stock > 0 THEN
        status_text := 'In Stock';
    ELSE
        status_text := 'Out of Stock';
    END IF;

    RETURN status_text;
END;
$$ LANGUAGE plpgsql;

-- How to call the function
SELECT get_product_status(1);

Example of an IF-THEN-ELSIF-ELSE block within a PL/pgSQL function.

3. Using COALESCE for Null Handling

While not a direct replacement for IF, the COALESCE function is extremely useful for handling NULL values conditionally. It returns the first non-NULL expression in a list. This can often simplify queries where you'd otherwise use a CASE statement just to check for NULL.

SELECT
    item_name,
    COALESCE(description, 'No description available') AS item_description
FROM
    items;

Using COALESCE to provide a default value for NULL descriptions.

Summary of Solutions

To avoid the syntax error at or near "IF", always choose the appropriate tool for your conditional logic in PostgreSQL:

1. For row-level conditional expressions within a query:

Use the CASE expression. This is the most common and idiomatic way to handle IF-THEN-ELSE logic directly in SELECT, WHERE, GROUP BY, or ORDER BY clauses.

2. For complex procedural logic (multiple statements, loops, variable assignments):

Define a PL/pgSQL function, stored procedure, or trigger. Within these blocks, you can freely use IF-THEN-ELSIF-ELSE statements.

3. For handling NULL values and providing defaults:

Utilize the COALESCE function. It's a concise way to replace NULL with a specified alternative.