Create date from day, month, year fields in MySQL

Efficiency optimization

To gain maximum speed with large datasets, prepare indexing on year_col, month_col, and day_col. Indices notably accelerate date-based queries and sorting tasks.

For columns disregarding the two-digit standard (like 1 for January instead of 01), use the LPAD() function for accurate date format:

-- SQL: The company you keep when you don't LPADDLE your own canoe 🛶
SELECT STR_TO_DATE(CONCAT(year_col, '-', LPAD(month_col, 2, '0'), '-', LPAD(day_col, 2, '0')), '%Y-%m-%d') AS date FROM your_table;
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Trade-offs might occur when contemplating alternative methods such as employing UNIX_TIMESTAMP() for conversion - it appears handy, but timestamp usage has potential timezone pains.

Crafting complex queries

MySQL simplifies the process for scenarios needing date comparisons. Suppose you need to extract records within a particular date range. The BETWEEN operator along with the structured date simplifies this task:

-- Who wants to live BETWEEN two dates? The only dates I care for are wrapped in bacon. 🥓
SELECT * FROM your_table 
WHERE STR_TO_DATE(CONCAT_WS('-', year_col, month_col, day_col), '%Y-%m-%d') 
BETWEEN '2021-01-01' AND '2021-12-31';
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Implicit versus explicit casting

MySQL is capable of performing implicit casting operations. In formats that are unambiguous, we can often omit explicit cast:

-- They say dating is hard. Try doing it in MySQL! 💌
SELECT CONCAT_WS('-', year_col, month_col, day_col) AS date FROM your_table 
WHERE CONCAT_WS('-', year_col, month_col, day_col) > '2021-01-01';
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MySQL implicitly understands the need for a date comparison and performs a cast accordingly.

Handling unique date formats

Occasionally, you might encounter date components in special formats. Thankfully, STR_TO_DATE() provides various patterns at your disposal:

-- Everyone has their unique style, just like every date format has its unique pattern! 🕵️‍♂️
SELECT STR_TO_DATE(CONCAT_WS('', day_col, month_col, year_col), '%d%m%Y') AS date FROM your_table;
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Here, the format assumed is dMY sans separators. Choosing the right pattern to match your date format is crucial.

Design implications

While designing a table with separate day, month, and year columns, have a holistic approach. For improved efficiency and data integrity, envision how the data will be manipulated and which operations will be prevalent.

Moreover, verify the final output of date object creation sans LPAD using a sample dataset. Run a few test cases ensuring MySQL's automatic type casting is acting as anticipated, averting potential issues later down the line.

Performance tuning

Review your queries' execution plans frequently, especially with tables bearing high read/writes. This insight might underscore the need for further indexing or hint towards possible opportunities for query refactoring to dodge full table scanning.