How to use a calculated column to calculate another column in the same view

sql

prompt-engineering

best-practices

performance

byAlex Kataev·Oct 24, 2024

Use a Common Table Expressions (WITH clause) to create a computation layer or inline the calculation in the final select. Compute the first calculated column, then reference it for the second calculation:

WITH CTE AS (
  SELECT
    base_column,
    (base_column * factor) AS first_calc  -- Let's do some math 🧮
  FROM
    table_name
)
SELECT
  base_column,
  first_calc,
  (first_calc + offset) AS second_calc  -- Let's do it again! 🔄
FROM
  CTE;

Here, first_calc is calculated in the CTE and reused in the main query for second_calc. This technique ensures correct sequencing of calculations while maintaining readability.

Crafting queries efficiently

Things can get messy with complex SQL computations. Here are the tools to keep it tidy:

Nested queries

If simplicity is the goal, a nested query does wonders:

SELECT
  outer_col,
  (SELECT AVG(inner_col) FROM inner_table WHERE ...) AS avg_val -- Nesting time! 🐣
FROM
  outer_table;

Cross apply in SQL Server

CROSS APPLY: Your handy sidekick when joining becomes a nuisance:

SELECT
  base_column,
  calc_column,
  applied_calc.*
FROM
  table_name
CROSS APPLY
  (SELECT (base_column + calc_column) * factor AS new_calc) AS applied_calc; -- Meet APPLY, less complex than JOINS 🤝

Optimal use of functions

Using functions? Choose ones that don't slow things down:

SELECT
  col1,
  expensive_function(col1) AS func_result,  -- i.e, not the function you take home to meet your parents 🙈
  func_result / factor AS final_result
FROM
  table_name;

The delicate art of performance optimization

Performance can make or break your SQL queries, especially with calculated columns:

When it pays to repeat calculations

If a calculation is light, it might not hurt to repeat it:

SELECT
  (col1 + col2) AS calc1,
  (calc1 / 2) + col3 AS calc2  -- Let's do the math tango one more time 🕺💃
FROM
  table_name;

Conditional play with case expressions

In SQL, we have case expressions where you set the calculation rules:

SELECT
  base_column,
  CASE
    WHEN base_column > 0 THEN (base_column * multiplier)  -- it's a "mult-ply" when > 0 🧮
    ELSE 0
  END AS conditional_calc
FROM
  table_name;

Advanced computation techniques

For off-the-beaten-path scenarios, SQL stands ready:

Leveraging APPLY in Oracle

It's called LATERAL in Oracle 12c+, but it's the same as SQL Server's APPLY:

SELECT
  t.column1,
  v.calculated_column
FROM
  table_name t,
  LATERAL (SELECT (t.column1 * factor) AS calculated_column FROM dual) v; -- APPLY's long-lost cousin, LATERAL 👯‍♂️

Use CTE with OFFSET/FETCH for pagination

With Common Table Expressions and OFFSET/FETCH, you get calculated columns and pagination all in one:

WITH RankedItems AS (
  SELECT
    ROW_NUMBER() OVER (ORDER BY base_column) AS rn,  -- SQL's version of musical chairs 🪑
    base_column,
    computation(base_column) AS calc
  FROM
    table_name
)
SELECT *
FROM RankedItems
WHERE rn BETWEEN @startRow AND @endRow
ORDER BY rn;

Validating calculations with sample data

Insert mock data using INSERT INTO to validate your view's calculations:

INSERT INTO table_name (base_column, ...)
VALUES (1, ...), (2, ...);  -- Insert mock data, not mocker of your data 😈

explain-codes / Sql / How to use a calculated column to calculate another column in the same view

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