thinQmaster Virtual testing partner Adaptive practice exams that find your weak spots and drill them. Every answer explained. From $69
Enroll

thinQmaster

Find your exam

Five questions free, before you decide.

Step 1 · choose a vendor

In one of our camps? Your practice exams come with the tuition — they are on your study page already.

Data & Analytics · Intermediate

Implicit Conversion

Automatic conversion of one data type to another by the database engine when an expression mixes types.

Definition

When types do not match, the engine converts one side according to type precedence. Implicit conversion on an indexed column can prevent index seeks and force scans, and it can cause errors or silent truncation.

Example

Comparing an NVARCHAR parameter to a VARCHAR indexed column can convert the column and cause a scan.

Exam tip

In performance-tuning questions, implicit conversion is a classic hidden reason why an index is not used.

Related terms