How do I combine 2 select statements into one?

Harnessing the Power of CASE

CASE statements let you perform conditional logic within a SELECT query. If you need values set based on specific conditions, turn to a CASE:

-- It's like a game! Condition met? You win 'value1'!
SELECT column,
       CASE
         WHEN condition THEN 'value1'
         ELSE 'value2'
       END AS new_column
FROM table;


In the rollercoaster of CASE, don't forget to mirror filter conditions to preserve data accuracy.

Balancing UNION and JOIN: A Tale of Performance

In the context of performance, the choice between UNIONs and JOINs is crucial. While UNION can be slower due to potential table scans; indexes can be your savior. A JOIN is usually zippier but needs interrelated data in the joined tables.

The Art of Subqueries and EXISTS

Subqueries can nest a SELECT within another, offering a twisted yet powerful path to data shaping:

-- Select-ception, a select within a select!
SELECT column
FROM table1
WHERE column EXISTS (
  SELECT 1
  FROM table2
  WHERE condition
);


Remember, with great power comes great responsibility. Subqueries can influence query performance, so tread carefully!

Visualization

Visualize you’re a chef with two recipes:

Recipe 1 (🍔): [Ingredient A, Ingredient B]
Recipe 2 (🌭): [Ingredient C, Ingredient D]

You're tasked with creating a combo meal using both recipes.

Combo Meal (🍱): [Ingredient A, Ingredient B] + [Ingredient C, Ingredient D]

In SQL, combining two SELECT statements is your version of creating a Combo Meal:

-- Bon Appétit SQL style!
SELECT * FROM Recipe1
UNION ALL
SELECT * FROM Recipe2;


The combo meal 🍱 serves ingredients from both 🍔 and 🌭.

Aliasing in SQL: The Game of Names

Aliases (AS) simplify your queries, especially when juggling with JOINS or complex queries:

-- Simple names for complex people
SELECT t1.column AS 'Table1_Column',
       t2.column AS 'Table2_Column'
FROM table1 t1
JOIN table2 t2 on t1.id = t2.id;


Hunting the Most Recent Data

The MAX() function helps you closer to the present by retrieving the latest record. It comes handy when dealing with timestamps:

-- MAX, because we don't like to live in the past
SELECT column, MAX(timestamp_column)
FROM table
GROUP BY column;


The Dynamism of SQL

Dynamic SQL provides a flexible path for crafting queries accommodating variable conditions. But SQL injection lurks in the shadows, so code safely!