How to Perform a LIKE Join in SQL: INSTR, LIKE, or CONCAT?

Performing a LIKE Join in SQL

When matching values between two tables using a join, there may be instances where a column from one table needs to match any part of a column from another table. This is often achieved using the LIKE operator in the join condition.

Let's say you have two tables, Table A and Table B, and you want to join them on a condition where columnA in Table A is "like" columnB in Table B. This means that the match could include the full value in columnB or characters before or after it.

Using INSTR:

One method is to use the INSTR() function in the join condition:

<code class="sql">SELECT *
FROM TABLE A
JOIN TABLE B ON INSTR(B.columnB, A.columnA) > 0</code>

Using LIKE:

Another option is to use the LIKE operator with wildcards:

<code class="sql">SELECT *
FROM TABLE A
JOIN TABLE B ON B.columnB LIKE '%'+ A.columnA +'%'</code>

Using LIKE with CONCAT:

You can also concatenate the wildcard symbols with the column value using the CONCAT() function:

<code class="sql">SELECT *
FROM TABLE A
JOIN TABLE B ON B.columnB LIKE CONCAT('%', A.columnA ,'%')</code>

Uppercase Conversion:

To ensure case-insensitive matches, it's recommended to convert the column values to uppercase before comparing them:

<code class="sql">SELECT *
FROM (SELECT UPPER(A.columnA) 'ua' FROM TABLE A) A
JOIN (SELECT UPPER(B.columnB) 'ub' FROM TABLE B) B ON INSTR(B.ub, A.ua) > 0</code>

Efficiency Considerations:

The efficiency of these methods may vary depending on the specific database and table structures. It's advisable to check the EXPLAIN plan output to determine the most optimal approach.

ANSI vs. Non-ANSI JOINs:

ANSI JOINs follow standardized syntax, while non-ANSI JOINs use a more traditional syntax. In this case, the JOIN clause is equivalent to the WHERE clause in a non-ANSI JOIN:

<code class="sql">SELECT *
FROM TABLE A,
     TABLE B
WHERE INSTR(B.columnB, A.columnA) > 0</code>

Using ANSI JOINs offers the advantage of separating the join condition from the filter condition in the WHERE clause.

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