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PostgreSQL

How to Comment in SQL

Database:

Operators:

Problem:

You'd like to make some comments in your SQL code.

Example:

Here's the example code:

SELECT
  name,
  COUNT(*) AS count_items
FROM products
GROUP BY name
HAVING COUNT(*) > 5
ORDER BY name DESC;

Solution 1:

Use --. Here is what it looks like:

-- product names along with the number of items
-- products ordered by name in descending order
SELECT
  name,
  COUNT(*) -- AS count_items
FROM products
GROUP BY name
-- HAVING COUNT(*) > 5
ORDER BY name DESC;

Discussion:

You can write single-line comments in SQL using --. After the --, you can write any comment you'd like as long as it's in one line. If you'd like to write a comment spanning multiple lines, you can use -- at the beginning of each comment line, just as in the example:

-- product names along with the number of items
-- products ordered by name in descending order

You can also use -- to comment out a whole line of code or a part of a line from some place in the middle of the line to the end of the line. For example, you can comment out AS count_items but leave the COUNT(*) in.

COUNT(*) -- AS count_items

Note that when you use --, you comment out the code to the end of the line, so you can’t use -- to comment out, for example, only the AS keyword. If you want to do this, take a look at Solution 2.

Solution 2:

Use /*...*/. Here is what it looks like:

/* product names along with the number of items
   this time, the products aren't sorted */
SELECT
  name,
  COUNT(*) /*AS*/ count_items
FROM products
GROUP BY name
/* HAVING COUNT(*) > 5
ORDER BY name DESC */;

Discussion:

/*...*/ is much more powerful than --. With /*...*/, you can comment out any part of the code you'd like. The comment can be multiline, and you can decide where it ends. Just write /* at the beginning of the comment, and */ at the end.

For example, you can comment out a single word from the code, like the AS keyword:

COUNT(*) /*AS*/ count_items

Or you can comment out two lines but still leave the semicolon at the end:

/* HAVING COUNT(*) > 5
ORDER BY name DESC */;

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