MySQL: "= true" vs "is true" on BOOLEAN. When is it advisable to use which one? And Which one is vendor independent?

MySQL is actually fooling you. It doesn't have a boolean column type at all:

BOOL, BOOLEAN

These types are synonyms for TINYINT(1). A value of zero is considered false. Nonzero values are considered true:

Also, the boolean literals are not such:

The constants TRUE and FALSE evaluate to 1 and 0, respectively.

Considering that:

• Many database systems do not have booleans either (not at least in standard SQL and column types)
• MySQL doesn't have an easy way to enforce 0 or 1 in BOOLEAN

My conclusion would be:

• You'll have to use WHERE IS flag or just WHERE flag because = simply doesn't work correctly. Which one, is possibly a matter of preference.
• Whatever you choose, no option will be vendor independent. For instance, Oracle won't even run either of them.

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Edit: if cross-platform is a must, I'd go for this:

WHERE flag=0
WHERE flag<>0

I'm sure we've all done it lots of times.

If the flag column is indexed and all values are either 0 or 1, where flag = true is much faster than where flag is true.

During our testing, is true resulted in a “full table scan” and took 1.121 seconds, while = true was executed with “key lookup” and only took 0.167 seconds. The table had about 3 million rows.