Why Does My SQL Query Fail with 'column ... does not exist' Due to Case Sensitivity?

PostgreSQL Case-Sensitivity and SQL Query Errors: "column ... does not exist"

Executing SQL joins can sometimes result in a frustrating "column 'column_name' does not exist" error. This often stems from a mismatch in case between the column name in your query and the actual, case-sensitive name defined in the database schema. PostgreSQL, unlike some other database systems, is particularly sensitive to this.

The Problem:

Consider this failing query:

<code class="language-sql">select sim.id as idsim, 
       num.id as idnum 
from main_sim sim 
  left join main_number num on (FK_Numbers_id=num.id);</code>

The error message points to the non-existent "FK_Numbers_id" column in the main_sim table.

The Solution:

The column does exist, but the correct capitalization is crucial. PostgreSQL's case-sensitive nature requires precise matching. The corrected query is:

<code class="language-sql">select sim.id as idsim, 
       num.id as idnum 
from main_sim sim 
  left join main_number num on ("FK_Numbers_id" = num.id);</code>

Why This Works:

Enclosing the column name "FK_Numbers_id" in double quotes forces PostgreSQL to treat it literally, regardless of case. Without the quotes, PostgreSQL interprets the column name according to its default case rules, leading to the error. Using double quotes ensures an exact match with the database schema's definition.

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