code: function columnsToSQL(columns, tables) {
    if (!columns) return
    const baseTable = Array.isArray(tables) && tables[0]
    let isDual = false
    if (baseTable && baseTable.type === 'dual') isDual = true
    return columns
      .map(column => {
        const { expr } = column
        if (isDual) expr.isDual = isDual
        let str = exprToSQL(expr)
        if (column.as !== null) {
          str = `${str} AS `
          if (column.as.match(/^[a-z_][0-9a-z_]*$/i)) str = `${str}${identifierToSql(column.as)}`
  
          else str = `${str}\`${column.as}\``
        }
  
        return str
      })
      .join(', ')
  }
docs: Stringify column expressions
  
  @param {Array} columns
  @return {string}