Why use '=' instead of ':='?

I am looking at the sample code for Go-SQL-Driver here:

//Insert
    stmt, err := db.Prepare("INSERT userinfo SET username=?")

// Update
    stmt, err = db.Prepare("update userinfo set username=?")

Why in Insert we use := but in Update we use =? It seems to me that both should be :=

:= is used in a short variable declaration; it both declares the variables on the left-hand-side, and assigns to them. (This is explained in the "Short variable declarations" section of The Go Programming Language Specification.)

=, by contrast, merely performs assignments.

In your example, the second line uses = because the variables have already been declared (by the first line), so := is not needed. (In fact, it will give an error message: := is only allowed when at least one of the variables is "new", i.e., not already declared.)

= is one of the assignement operators.

:= is the short variable declaration.

In the referenced links you can find that the semantics is quite different, so actually the concept "instead of" is a bit problematic.

A perhaps helpful mnemotechnic rule:

identifierI := expressionE

is the same as

var identifierI = expressionE

which is the same as

var identifierI typeOf(expressionE)
identifierI = expressionE

:= is for convenience. The important difference is that the := does type inference, so as it declares and assigns the variable all one line, the variables type is inferred from the return value's type of the function.

This makes your program easier to read in most cases but does mean someone will have to look up in the docs the function's return value type to figure out the variable's type.

You'll want to use = when you're re-assiging to an existing variable or when assigning to a global/package variable from within a function, otherwise, you'll be creating a new local variable.