Understanding SQL Query Differences: A Comprehensive Guide

When working with SQL (Structured Query Language), it's essential to understand the differences in syntax and how these differences impact query execution. One common confusion arises with the JOIN keyword and its alternatives. This article will explore the nuances of these SQL nuances, explaining the differences in syntax and their implications on query performance and standardization.

SQL Joins and Query Syntax

Practically speaking, there is no significant difference in speed between foo bar and foo JOIN bar. Both are valid syntaxes for performing joins in SQL, and the DBMS (Database Management System) will handle them equivalently. The choice of syntax is more about style and compatibility with legacy systems.

Understanding the Syntax: Historical Background

The syntax foo bar is an older way of writing joins in SQL. This format was prevalent before the modern JOIN keyword was standardized. foo JOIN bar is the newer, ANSI standard. Both formats are interpreted similarly by modern DBMS, and the choice between them does not affect query performance.

The Role of the Database Engine

The difference lies in how the database engine interprets and calculates the execution plan for the query. The EXPLAIN utility can be used to show the execution plan, which is the same for both syntaxes:

EXPLAIN SELECT * FROM t1, t2 WHERE t1.c1 t2.c1

EXPLAIN SELECT * FROM t1 JOIN t2 ON t1.c1 t2.c1

Both commands will generate the same execution plan, which means they will perform the same operations in the same manner.

Inner and Outer Joins: Database System Dependence

When it comes to inner and outer joins, the syntax JOIN and foo bar can be used interchangeably, but the interpretation might vary depending on the database system. For example, in some database systems, t1, t2 might be interpreted as an outer join, while in others it might be an inner join. This can lead to confusion if the database system is not the same for all developers.

Standardized Code Practices

To ensure standardized and clear code, it is advisable to use the JOIN syntax. This practice promotes better readability and maintains consistency across different database systems. Additionally, using JOIN explicitly makes the intent of the query more clear to anyone reading the code.

Example: MariaDB Behavior

Consider the following example in MariaDB, which is similar to MySQL:

Creating tables:

CREATE TABLE t1 (c1 INT);
CREATE TABLE t2 (c2 INT, c1 INT);

Inserting data:

INSERT INTO t1 VALUES (1);
INSERT INTO t2 VALUES (1, 11);

Explain the query:

EXPLAIN EXTENDED SELECT * FROM t1, t2 WHERE t2.c1  t1.c1;

Output explanation:

ID        | SELECT TYPE | TABLE   | TYPE | POSSIBLE KEYS | KEY  | KEY LENGTH | REF  | ROWS | FILTERED | EXTRA
-------------------------------------------------------------------------------------------------------------------------------------
1         | SIMPLE      | t1      | ALL  | NULL          | NULL | NULL       | NULL | 1    | 100.00   |
1         | SIMPLE      | t2      | ALL  | NULL          | NULL | NULL       | NULL | 1    | 100.00   | Using where; Using join buffer; Flat BNL join

The EXPLAIN utility shows that both syntaxes generate the same execution plan, and the second query clearly uses the JOIN keyword, though the combined table syntax is interpreted as an inner join.

Conclusion

In summary, there is no practical difference in performance between using foo bar and foo JOIN bar for join operations. Both are valid and interpreted similarly by modern DBMS. However, to ensure clarity and maintainability across different database systems, it is advisable to use the ANSI standard syntax JOIN. This practice not only improves code readability but also adheres to established standards, making your queries more robust and easier to understand.