无论我尝试什么包和格式，我的表都无法居中。
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{graphicx}
\usepackage[export]{adjustbox}
\usepackage{multirow}
\usepackage{booktabs}
\title{xxxxx}
\author{xxxxxxxx }
\date{July 2019}
\begin{document}
\maketitle
\begin{center}
{
\scalebox{.79}{{
\centering
\begin{tabular}{| ccccccccc |}
\toprule
Metric Date & Metric Values & Metric Rank & VIX Date & VIX Values & VIX Rank & metric rank & vix rank & timedelta \\
\midrule
2017-06-20 & 0.3217 & 1 & 2018-02-05 & 37.32 & 1 & 10 & 1 & 0 days \\
2016-12-19 & 0.3194 & 2 & 2018-12-24 & 36.07 & 2 & 10 & 6 & 1 days \\
2017-09-18 & 0.3193 & 3 & 2018-02-08 & 33.46 & 3 & 9 & 9 & 1 days \\
2016-12-27 & 0.2993 & 4 & 2018-12-26 & 30.41 & 4 & 10 & 3 & 3 days \\
2017-06-05 & 0.2874 & 5 & 2018-12-27 & 30.41 & 5 & 10 & 8 & 4 days \\
2017-05-23 & 0.2708 & 6 & 2018-02-06 & 29.98 & 6 & 9 & 2 & 5 days \\
2017-08-25 & 0.2461 & 7 & 2018-12-28 & 29.96 & 7 & 9 & 4 & 7 days \\
2017-08-24 & 0.2398 & 8 & 2018-02-09 & 29.06 & 8 & 9 & 5 & 8 days \\
2018-12-19 & 0.2391 & 9 & 2018-12-20 & 28.38 & 9 & 9 & 7 & 9 days \\
2018-02-05 & 0.2317 & 10 & 2016-02-11 & 28.14 & 10 & 3 & 1 & 140 days \\
\bottomrule
\end{tabular}}
}}\end{center}
\end{document}
答案1
缩放表格应该是最后的手段。.79缩放系数只是一个大胆的（也是错误的）猜测，它留下了一个过大的对象，当然它不居中，并且突出在右边距。
首先修复浪费大量空间的长标题；然后压缩列间空间；使用起来很方便tabular*，将计算留给 TeX 来完成。
我留下了（注释掉）\small。尝试一下，看看表格是否看起来更好。
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{graphicx}
\usepackage[export]{adjustbox}
\usepackage{multirow}
\usepackage{booktabs}
\title{xxxxx}
\author{xxxxxxxx }
\date{July 2019}
\begin{document}
\maketitle
\begin{center}
%\small
\setlength{\tabcolsep}{0pt}
\begin{tabular*}{\textwidth}{@{\extracolsep{\fill}}ccccccccc@{}}
\toprule
\multicolumn{3}{c}{Metric} & \multicolumn{3}{c}{VIX} & \multicolumn{2}{c}{rank} & timedelta \\
\cmidrule{1-3} \cmidrule{4-6} \cmidrule{7-8}
Date & Values & Rank & Date & Values & Rank & metric & vix\\
\midrule
2017-06-20 & 0.3217 & 1 & 2018-02-05 & 37.32 & 1 & 10 & 1 & 0 days \\
2016-12-19 & 0.3194 & 2 & 2018-12-24 & 36.07 & 2 & 10 & 6 & 1 days \\
2017-09-18 & 0.3193 & 3 & 2018-02-08 & 33.46 & 3 & 9 & 9 & 1 days \\
2016-12-27 & 0.2993 & 4 & 2018-12-26 & 30.41 & 4 & 10 & 3 & 3 days \\
2017-06-05 & 0.2874 & 5 & 2018-12-27 & 30.41 & 5 & 10 & 8 & 4 days \\
2017-05-23 & 0.2708 & 6 & 2018-02-06 & 29.98 & 6 & 9 & 2 & 5 days \\
2017-08-25 & 0.2461 & 7 & 2018-12-28 & 29.96 & 7 & 9 & 4 & 7 days \\
2017-08-24 & 0.2398 & 8 & 2018-02-09 & 29.06 & 8 & 9 & 5 & 8 days \\
2018-12-19 & 0.2391 & 9 & 2018-12-20 & 28.38 & 9 & 9 & 7 & 9 days \\
2018-02-05 & 0.2317 & 10 & 2016-02-11 & 28.14 & 10 & 3 & 1 & 140 days \\
\bottomrule
\end{tabular*}
\end{center}
\end{document}
需要进一步减少列宽：删除重复信息总是好的。感谢 Fran 的建议。
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{graphicx}
\usepackage[export]{adjustbox}
\usepackage{multirow}
\usepackage{booktabs}
\title{xxxxx}
\author{xxxxxxxx }
\date{July 2019}
\begin{document}
\maketitle
\begin{center}
%\small
\setlength{\tabcolsep}{0pt}
\begin{tabular*}{\textwidth}{@{\extracolsep{\fill}}ccccccccc@{}}
\toprule
\multicolumn{3}{@{}c}{Metric} & \multicolumn{3}{c}{VIX} & \multicolumn{2}{c}{rank} &
\smash{\begin{tabular}[t]{c} time $\delta$ \\ (days) \end{tabular}} \\
\cmidrule{1-3} \cmidrule{4-6} \cmidrule{7-8}
Date & Values & Rank & Date & Values & Rank & metric & vix\\
\midrule
2017-06-20 & 0.3217 & 1 & 2018-02-05 & 37.32 & 1 & 10 & 1 & 0 \\
2016-12-19 & 0.3194 & 2 & 2018-12-24 & 36.07 & 2 & 10 & 6 & 1 \\
2017-09-18 & 0.3193 & 3 & 2018-02-08 & 33.46 & 3 & 9 & 9 & 1 \\
2016-12-27 & 0.2993 & 4 & 2018-12-26 & 30.41 & 4 & 10 & 3 & 3 \\
2017-06-05 & 0.2874 & 5 & 2018-12-27 & 30.41 & 5 & 10 & 8 & 4 \\
2017-05-23 & 0.2708 & 6 & 2018-02-06 & 29.98 & 6 & 9 & 2 & 5 \\
2017-08-25 & 0.2461 & 7 & 2018-12-28 & 29.96 & 7 & 9 & 4 & 7 \\
2017-08-24 & 0.2398 & 8 & 2018-02-09 & 29.06 & 8 & 9 & 5 & 8 \\
2018-12-19 & 0.2391 & 9 & 2018-12-20 & 28.38 & 9 & 9 & 7 & 9 \\
2018-02-05 & 0.2317 & 10 & 2016-02-11 & 28.14 & 10 & 3 & 1 & 140 \\
\bottomrule
\end{tabular*}
\end{center}
\end{document}
