sql: fix create database reparse issue

The create database test were having reparse issues
because the syntax requires an integer constant for
the CONNECTION LIMIT parameter. This change replaces
the underscore with an integer.

Release note: None

pkg/sql/parser/testdata/create_database

@@ -128,17 +128,15 @@ CREATE DATABASE a CONNECTION LIMIT = 13
 ----
 CREATE DATABASE a CONNECTION LIMIT = 13
 CREATE DATABASE a CONNECTION LIMIT = 13 -- fully parenthesized
-CREATE DATABASE a CONNECTION LIMIT = _ -- literals removed
-REPARSE WITHOUT LITERALS FAILS: at or near "_": syntax error
+CREATE DATABASE a CONNECTION LIMIT = 0 -- literals removed
 CREATE DATABASE _ CONNECTION LIMIT = 13 -- identifiers removed
 
 parse
 CREATE DATABASE a WITH CONNECTION LIMIT = 13
 ----
 CREATE DATABASE a CONNECTION LIMIT = 13 -- normalized!
 CREATE DATABASE a CONNECTION LIMIT = 13 -- fully parenthesized
-CREATE DATABASE a CONNECTION LIMIT = _ -- literals removed
-REPARSE WITHOUT LITERALS FAILS: at or near "_": syntax error
+CREATE DATABASE a CONNECTION LIMIT = 0 -- literals removed
 CREATE DATABASE _ CONNECTION LIMIT = 13 -- identifiers removed
 
 parse

pkg/sql/sem/tree/create.go

@@ -94,7 +94,7 @@ func (node *CreateDatabase) Format(ctx *FmtCtx) {
 	if node.ConnectionLimit != -1 {
 		ctx.WriteString(" CONNECTION LIMIT = ")
 		if ctx.flags.HasFlags(FmtHideConstants) {
-			ctx.WriteByte('_')
+			ctx.WriteByte('0')
 		} else {
 			// NB: use ctx.FormatNode when the connection limit becomes an expression.
 			ctx.WriteString(strconv.Itoa(int(node.ConnectionLimit)))